Validity and reliability of an instrument to measure factors associated with screen time in Brazilian students

Barbosa Filho, Valter Cordeiro; Bandeira, Alexsandra Silva; Rech, Cassiano Ricardo; Lopes, Adair Silva; Mota, Jorge; Silva, Kelly Samara

doi:10.1590/1413-81232021263.31322018

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FREE THEMES • Ciênc. saúde coletiva 26 (3) • Mar 2021 • https://doi.org/10.1590/1413-81232021263.31322018 copy

Validity and reliability of an instrument to measure factors associated with screen time in Brazilian students

Validade e confiabilidade de um instrumento para avaliar fatores associados ao tempo de tela em escolares brasileiros

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

This study analyzed the validity and reliability of an instrument with scales measuring factors associated with screen time in adolescents. Two cross-sectional studies included students for analyses of validity and internal consistency (n=1138, 51.5% boys) and test-retest reproducibility (n=194, 56.2% boys). Individual (attitude, self-efficacy and expectations), social (behaviors, rules, beliefs and family support and support of school members) and physical environmental (home and bedroom equipment) factors of screen time were evaluated. All the scales of the instrument met the statistical criteria of the validity of the exploratory factorial analysis (p<0.001). The exploratory factorial analysis indicated five one-dimensional and three two-factor scales (self-efficacy, expectations and familiar behavior), with factorial loads of 0.56 to 0.88. There were significant correlations between the scores of almost all the scales and the reported screen time (p<0.05). Eight of nine scales/subscales showed Cronbach’s α value being higher than 0.70, and ICC ranged from 0.63 to 0.83. In conclusion, the instrument presented scales with acceptable validity and reliability and can be used to measure factors associated with screen time in Brazilian students.

Key words
Sedentary lifestyles; Associated factors; Questionnaire; Validity tests; Reproducibility of results

Factors associated with screen time use	Description and instruments used as reference	n of itens	Scale
Scales of individual variables
Attitude for reducing screen time use^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	Students' perception of affective and instrumental aspects that are generally related to the behavior in focus¹³13 Norman GJ, Vaughn AA, Roesch SC, Sallis JF, Calfas KJ, Patrick K. Development of decisional balance and self-efficacy measures for adolescent sedentary behaviors. Psychol Health 2004; 19(5):561-575.; in this instrument, the reduction of the screen time use.	3	A four-point semantic differential scale with binary pairs of adjectives (trivial/important, boring/fun, harmful/health)
Self-efficacy for reducing screen time use^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	Students' perception of abilities (8 items) and confidence (3 items) for reducing their screen time use¹⁰10 Gebremariam MK, Totland TH, Andersen LF, Bergh IH, Bjelland M, Grydeland M, Ommundsen Y, Lien N. Stability and change in screen-based sedentary behaviours and associated factors among Norwegian children in the transition between childhood and adolescence. BMC Public Health 2012; 12:104.^,¹²12 Jago R, Baranowski T, Watson K, Bachman C, Baranowsi JC, Thompson D, Hernández AE, Venditti E, Blackshear T, Moe E. Development of new physical activity and sedentary behavior change self-efficacy questionnaires using item response modeling. Int J Behav Nutr Phys Act 2009; 6:20.^,¹³13 Norman GJ, Vaughn AA, Roesch SC, Sallis JF, Calfas KJ, Patrick K. Development of decisional balance and self-efficacy measures for adolescent sedentary behaviors. Psychol Health 2004; 19(5):561-575..	11	Likert with 4 points, 1"Strongly disagree" to 4 "Strongly agree"
Negative and positive expectations of screen time use^ Only the scores of negative expectations of screen time use subscale were inverted.	Students' perception on the positive (8 items) and negative (4 items) expectations about their screen time use¹³13 Norman GJ, Vaughn AA, Roesch SC, Sallis JF, Calfas KJ, Patrick K. Development of decisional balance and self-efficacy measures for adolescent sedentary behaviors. Psychol Health 2004; 19(5):561-575.^,¹⁴14 Salmon J, Hume C, Ball K, Booth M, Crawford D. Individual, social and home environment determinants of change in children's television viewing: The Switch-Play intervention. J Sci Med Sport 2006; 9(5):378-387.^,¹⁷17 Norman GJ. Psychosocial and Environmental Correlates of Adolescent Sedentary Behaviors. Pediatrics 2005; 116(4):908-916..	12	Likert with 4 points, 1"Strongly disagree" to 4 "Strongly agree"
Scales of social environmental variables
Family screen time use	Students' perception of how often the parents and other family members use TV and computer/video games, considering individual use (4 items) and together with them (4 items)¹⁴14 Salmon J, Hume C, Ball K, Booth M, Crawford D. Individual, social and home environment determinants of change in children's television viewing: The Switch-Play intervention. J Sci Med Sport 2006; 9(5):378-387.^,¹⁵15 Tandon PS, Zhou C, Sallis JF, Cain KL, Frank LD, Saelens BE. Home environment relationships with children's physical activity, sedentary time, and screen time by socioeconomic status. Int J Behav Nutr Phys Act 2012; 9(1):88..	8	Never, rarely, frequently, always
Family rules of screen time use^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	Students' perception of how often a household member gave different types of rules for screen time use (e.g., limit screen use to only 2 hours per day)¹¹11 Hardy LL, Baur LA, Garnett SP, Crawford D, Campbell KJ, Shrewsbury VA, Cowell CT, Salmon J. Family and home correlates of television viewing in 12-13 year old adolescents: the Nepean Study. Int J Behav Nutr Phys Act 2006; 3:24.^,¹⁵15 Tandon PS, Zhou C, Sallis JF, Cain KL, Frank LD, Saelens BE. Home environment relationships with children's physical activity, sedentary time, and screen time by socioeconomic status. Int J Behav Nutr Phys Act 2012; 9(1):88..	6	Never, rarely, frequently, always
Family beliefs on screen time use	Students' perception of family beliefs on screen time use¹¹11 Hardy LL, Baur LA, Garnett SP, Crawford D, Campbell KJ, Shrewsbury VA, Cowell CT, Salmon J. Family and home correlates of television viewing in 12-13 year old adolescents: the Nepean Study. Int J Behav Nutr Phys Act 2006; 3:24.^,¹⁵15 Tandon PS, Zhou C, Sallis JF, Cain KL, Frank LD, Saelens BE. Home environment relationships with children's physical activity, sedentary time, and screen time by socioeconomic status. Int J Behav Nutr Phys Act 2012; 9(1):88.^,¹⁷17 Norman GJ. Psychosocial and Environmental Correlates of Adolescent Sedentary Behaviors. Pediatrics 2005; 116(4):908-916..	3	Likert with 4 points, 1"Strongly disagree" to 4 "Strongly agree"
Family support for reducing screen time use^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	Students' perception of support for reducing screen time use from the household members¹¹11 Hardy LL, Baur LA, Garnett SP, Crawford D, Campbell KJ, Shrewsbury VA, Cowell CT, Salmon J. Family and home correlates of television viewing in 12-13 year old adolescents: the Nepean Study. Int J Behav Nutr Phys Act 2006; 3:24.^,¹⁵15 Tandon PS, Zhou C, Sallis JF, Cain KL, Frank LD, Saelens BE. Home environment relationships with children's physical activity, sedentary time, and screen time by socioeconomic status. Int J Behav Nutr Phys Act 2012; 9(1):88.^,¹⁷17 Norman GJ. Psychosocial and Environmental Correlates of Adolescent Sedentary Behaviors. Pediatrics 2005; 116(4):908-916..	5	Likert with 4 points, 1"Strongly disagree" to 4 "Strongly agree"
School members support for reducing screen time use^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	Students' perception of support for reducing screen time use from school members (friends, teachers, coordinators)¹¹11 Hardy LL, Baur LA, Garnett SP, Crawford D, Campbell KJ, Shrewsbury VA, Cowell CT, Salmon J. Family and home correlates of television viewing in 12-13 year old adolescents: the Nepean Study. Int J Behav Nutr Phys Act 2006; 3:24.^,¹⁵15 Tandon PS, Zhou C, Sallis JF, Cain KL, Frank LD, Saelens BE. Home environment relationships with children's physical activity, sedentary time, and screen time by socioeconomic status. Int J Behav Nutr Phys Act 2012; 9(1):88.^,¹⁷17 Norman GJ. Psychosocial and Environmental Correlates of Adolescent Sedentary Behaviors. Pediatrics 2005; 116(4):908-916..	4	Likert with 4 points, 1"Strongly disagree" to 4 "Strongly agree"
Items of physical environmental variables
Home and bedroom screen equipment	Items (TV, computer with and without internet access, and video games) in their home and in their bedroom that may encourage screen time use¹¹11 Hardy LL, Baur LA, Garnett SP, Crawford D, Campbell KJ, Shrewsbury VA, Cowell CT, Salmon J. Family and home correlates of television viewing in 12-13 year old adolescents: the Nepean Study. Int J Behav Nutr Phys Act 2006; 3:24.^,¹⁴14 Salmon J, Hume C, Ball K, Booth M, Crawford D. Individual, social and home environment determinants of change in children's television viewing: The Switch-Play intervention. J Sci Med Sport 2006; 9(5):378-387..	8	Number of items (0, 1, 2, and 3 or more)

Descriptive Variables	Sample of validation and internal consistency analyzes (n=1,138)		Sample of test–retest reproducibility analyzes (n=194)		X²2 Chastin SFM, De Craemer M, Lien N, Bernaards C, Buck C, Oppert J-M, Nazare J-A, Lakerveld J, O'Donoghue G, Holdsworth M, Owen N, Brug J, Cardon G, DEDIPAC consortium, expert working group and consensus panel. The SOS-framework (Systems of Sedentary behaviours): an international transdisciplinary consensus framework for the study of determinants, research priorities and policy on sedentary behaviour across the life course: a DEDIPAC-study. Int J Behav Nutr Phys Act 2016; 13:83.	p-value
Descriptive Variables	n	%	n	%		p-value
Gender					1.46	0.226
Boys	558	51.5	109	56.2
Girls	526	48.5	85	43.8
Age groups (years)					0.094	0.760
11-12	169	14.9	34	17.6
13-14	642	56.6	101	52.3
15	204	18.0	40	20.7
16 or more	120	10.6	18	9.3
Grades					6.561	0.065
7^th	474	41.7	71	36.6
8^th	400	35.1	73	37.6
9^th	264	23.2	50	25.8
Reported TV time use					1.96	0.187
<2 hours/day	374	29.4	57	29.4
≥2 hours/day	710	70.6	137	70.6
Reported computer/games time use
<2 hours/day	556	51.3	70	36.1	15.43	<0.001
≥2 hours/day	528	48.7	124	63.9

Scales (n items and % of variance explained by these items)	Description	Factor loading		Reliability
Scales (n items and % of variance explained by these items)	Mean (SD)	1	2	α^ Cronbach's α was used to represent the internal consistency of the total scale and excluding each item.	ICC	95%CI
Scales and subscales of individual aspects of screen time use
Attitude for reducing screen time use (3 items, 57.9%)	7.40 (2.08)	KMO=0.63 Bartlett's test p<0.01		0.64	0.71	0.63	0.79
Unimportant/important	2.50 (1.00)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.81	-		0.72	0.64	0.81
Boring/Funny	2.89 (0.83)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.73	-		0.70	0.62	0.79
Bad/good for health	2.01 (0.90)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.74	-		0.68	0.60	0.77
Self-efficacy for reducing screen time use (11items, 49.6%)	27.29 (5.69)	KMO=0.88 Bartlett's test p<0.01		0.84	0.63	0.55	0.72
Individual limits and abilities (8 items, 38.8%)	20.22 (4.76)	KMO=0.88 Bartlett's test p<0.01		0.85	0.71	0.63	0.80
To limit time watching TV for 2 hours a day	2.34 (0.82)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.67	-		0.65	0.57	0.74
To turn off the TV	2.80 (0.84)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.69	-		0.85	0.77	0.94
Do not watch TV on most school days	2.46 (0.82)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.59	-		0.64	0.56	0.73
To limit time using computer/games for 2 hours a day	2.49 (0.86)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.76	-		0.65	0.57	0.74
To turn off computer/games	2.71 (0.87)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.76	-		0.73	0.65	0.82
Do not use computer/games on school days	2.51 (0.86)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.70	-		0.69	0.61	0.78
To determine limits in screen time use	2.44 (0.83)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.73	-		0.66	0.58	0.75
To change screen time for physical activity in free-time	2.48 (0.94)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.66	-		0.73	0.65	0.82
Confidence (3 items, 10.7%)	7.07 (1.68)			0.61	0.64	0.56	0.73
To schedule the time watching TV on weekdays	2.38 (0.75)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.78		0.66	0.58	0.75
To watch TV doing other activities	2.38 (0.80)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.56		0.61	0.53	0.70
To schedule the time using computer/games on weekdays	2.30 (0.82)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.78		0.63	0.55	0.72
Expectations with screen time use (12 items, 50.7%)	31.63 (4.98)	KMO=0.86 Bartlett's test p<0.01		0.71	0.74	0.66	0.83
Positive expectations (8 items, 33.5%)	22.29 (4.65)	KMO=0.86 Bartlett's test p<0.01		0.85	0.78	0.70	0.87
It is relaxing to watch TV	2.86 (0.76)	0.56	-		0.69	0.61	0.78
I feel good (happy) when I am using computer/games	3.03 (0.75)	0.76	-		0.64	0.56	0.73
I get excited (agitated) when I am using computer/games	2.79 (0.83)	0.72	-		0.69	0.61	0.78
Connecting with friends using computer/games	2.84 (0.85)	0.73	-		0.70	0.62	0.79
Cut the time using computer is not good to my friends	2.54 (0.88)	0.57	-		0.66	0.58	0.75
I like to use screen for many hours at the same time	2.74 (0.88)	0.79	-		0.76	0.68	0.85
I prefer to use screen during free-time	2.91 (0.84)	0.77	-		0.67	0.59	0.76
I use screen to escape the world (e.g., problems)	2.59 (0.90)	0.62	-		0.61	0.53	0.70
Negative expectations (4 items, 17.2%)	9.34 (2.58)			0.70	0.65	0.57	0.74
It trap me from doing important things (e.g., studying)	2.26 (0.85)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.68		0.62	0.54	0.71
I get lazy	2.22 (0.86)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.72		0.58	0.50	0.67
I feel pain in the body (back, legs)	2.45 (0.89)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.79		0.69	0.61	0.78
It burns my eyes and leaves me with a headache	2.41 (0.91)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	-	0.74		0.66	0.58	0.75
Scales and subscales of social environmental variables
Family model of screen time use (8 items, 55.5%)	18.39 (4.59)	KMO=0.64 Bartlett's test p<0.01		0.73	0.74	0.66	0.83
TV use of household members (4 items, 34.8%)	11.06 (2.94)	KMO=0.64 Bartlett's test p<0.01		0.74	0.70	0.62	0.79
Parents' TV watching	3.00 (0.96)	0.73	-		0.69	0.61	0.78
Household's TV watching	3.03 (0.98)	0.74	-		0.73	0.65	0.82
Parents and son/daughter watching TV together	2.50 (1.00)	0.77	-		0.70	0.62	0.79
Households and son/daughter watching TV together	2.53 (1.00)	0.76	-		0.67	0.59	0.76
Computer/games use of household members (4 items, 20.7%)	7.32 (2.83)			0.70	0.64	0.56	0.73
Parents' computer/games using	1.75 (0.96)	-	0.69		0.72	0.64	0.81
Household's computer/games using	2.36 (1.15)	-	0.67		0.60	0.52	0.69
Parents and son/daughter using computer/games together	1.45 (0.80)	-	0.79		0.65	0.57	0.74
Households and son/daughter using computer/games together	1.77 (0.96)	-	0.76		0.68	0.60	0.77
Family rules of screen time use (6 items, 57.6%)	18.80 (4.69)	KMO=0.84 Bartlett's test p<0.01		0.85	0.69	0.61	0.78
Limiting TV time	3.17 (0.99)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.72	-		0.66	0.58	0.75
Observe the TV content	2.84 (1.09)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.69	-		0.64	0.56	0.73
Turn off TV	3.27 (1.00)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.79	-		0.62	0.54	0.71
Limiting computer/games use	3.19 (1.02)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.81	-		0.72	0.64	0.81
Observe the computer/games content	3.04 (1.06)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.75	-		0.58	0.50	0.67
Turn off computer/games	3.30 (1.01)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.79	-		0.66	0.58	0.75
Family beliefs of screen time use (3 items, 52.9%)	7.57 (1.87)	KMO=0.62 Bartlett's test p<0.01		0.55	0.66	0.58	0.75
Using screen is cool/fun	2.66 (0.81)	-	0.68		0.53	0.45	0.62
Using screen is better than perform physical activity.	2.38 (0.85)	-	0.77		0.58	0.50	0.67
Using screen is safer than perform physical activity outside the home.	2.53 (0.91)	-	0.74		0.59	0.51	0.68
Family support for reducing screen time use (5 items, 54.2%)	12.46 (3.19)	KMO=0.83 Bartlett's test p<0.01		0.78	0.67	0.59	0.76
To encourage to decrease screen time	2.55 (0.85) ^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.74	-		0.55	0.47	0.64
To speak that screen time is unhealthy	2.23 (0.84) ^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.76	-		0.62	0.54	0.71
To help to decrease screen time	2.54 (0.83) ^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.80	-		0.54	0.46	0.63
To laud reducing screen time	2.54 (0.89) ^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.75	-		0.70	0.62	0.79
To limit screen time when I do something wrong.	2.60 (0.93) ^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.64	-		0.63	0.55	0.72
School members support for reducing screen time use (4 items, 68.7%)	11.20 (2.93)	KMO=0.81 Bartlett's test p<0.01		0.85	0.69	0.61	0.78
To encourage to decrease screen time	2.90 (0.88)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.83	-		0.65	0.57	0.74
To speak that screen time is unhealthy	2.62 (0.90)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.83	-		0.62	0.54	0.71
To help to decrease screen time	2.80 (0.86)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.88	-		0.59	0.51	0.68
To laud reducing screen time	2.89 (0.90)^* * These item scales were inverted in order to standardize all scale scores (higher scores were favorable to screen time use).	0.78	-		0.70	0.62	0.79
Items of physical environmental variables
Number of TV at home	2.00 (0.76)				0.78	0.70	0.87
Number of Video games at home	0.53 (0.64)				0.81	0.73	0.90
Number of Computer with internet access at home	0.98 (0.96)				0.72	0.64	0.81
Number of Computer without internet access at home	0.22 (0.53)				0.66	0.58	0.75
Number of TV in bedroom	0.65 (0.52)				0.76	0.68	0.85
Number of Video games in bedroom	0.30 (0.50)				0.83	0.75	0.92
Number of Computer with internet access in bedroom	0.55 (0.71)				0.71	0.63	0.80
Number of Computer without internet access in bedroom	0.13 (0.39)				0.70	0.62	0.79

Scales	Reported daily screen time use in leisure-time
Scales	Total screen time use	TV time use	Computer/games time use
Scales and subscales of individual aspects of screen time use
Attitude for reducing screen time use	0.255^ p<0.01.	0.180^ p<0.01.	0.198^ p<0.01.
Self-efficacy for reducing screen time use	0.200^ p<0.01.	0.157^ p<0.01.	0.136^ p<0.01.
Individual limits and abilities	0.335^ p<0.01.	0.209^ p<0.01.	0.280^ p<0.01.
Confidence	-0.068^* * p<0.05;	-0.016	-0.090^ p<0.01.
Expectations with screen time use	0.393^ p<0.01.	0.196^ p<0.01.	0.392^ p<0.01.
Positive expectations	0.462^ p<0.01.	0.234^ p<0.01.	0.447^ p<0.01.
Negative expectations	0.104^ p<0.01.	0.025	0.131^ p<0.01.
Scales and subscales of social environmental variables
Family model of screen time use	0.283^ p<0.01.	0.225^ p<0.01.	0.235^ p<0.01.
TV use of household members	0.166^ p<0.01.	0.287^ p<0.01.	0.001
Computer/games use of household members	0.236^ p<0.01.	0.013	0.359^ p<0.01.
Family rules of screen time use	0.001	0.003	-0.014
Family beliefs of screen time use	0.154^ p<0.01.	0.154^ p<0.01.	0.098^ p<0.01.
Family support for reducing screen time use	0.010	0.052	-0.031
School members support for reducing screen time use	0.035	0.007	0.042
Items of physical environmental variables (screen equipment)
Number of TV at home	0.127^ p<0.01.	0.129^ p<0.01.	0.080^ p<0.01.
Number of Video games at home	0.223^ p<0.01.	0.081^ p<0.01.	0.297^ p<0.01.
Number of Computer with internet access at home	0.244^ p<0.01.	-0.002	0.364^ p<0.01.
Number of Computer without internet access at home	-0.015	0.024	-0.028
Number of TV in bedroom	0.109^ p<0.01.	0.134^ p<0.01.	0.055
Number of Video games in bedroom	0.215^ p<0.01.	0.103^ p<0.01.	0.248^ p<0.01.
Number of Computer with internet access in bedroom	0.235^ p<0.01.	0.003	0.348^ p<0.01.
Number of Computer without internet access in bedroom	-0.006	-0.011	0.008

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E-mail: cienciasaudecoletiva@fiocruz.br

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[1] Collaborations

VC Barbosa Filho participated in all stages of the study, including design, data collection and analyses, and he drafted the first version of the manuscript. AS Bandeira participated in study design, data collection and she also drafted the first version of the manuscript. CR Rech, J Mota, AS Lopes and KS Silva participated in the study design, data interpretation and the review of the manuscript. All authors approved the final version of the manuscript.

Attitude toward reducing screen time use	Give your opinion on the following statements:
	Decreasing the time in front of the TV, computer or video games is important for you.	Not important; A little important; Important; Very important
	Decreasing the time in front of the TV, computer or video games is either boring or fun for you.	Very boring; Boring; Fun; Great fun
	Decreasing the time in front of the TV, computer or video games is good or bad for your health.	Very bad; Bad; Good; Very good
Self-efficacy toward reducing screen time use	I think I am able...
	Individual limits and abilities
	...to limit (decrease) my time watching TV for two hours a day.	Strongly Disagree; Disagree; Agree; Strongly Agree
	...to turn off the TV, even though I am watching a program that I like.
	...not to watch TV on most days with school classes.
	...to limit (decrease) my time using the computer/video games for two hours a day.
	...to turn off the computer/video game, even though I'm doing something I enjoy (favorite games, chatting).
	...not to not use computer/video games on most days with school classes.
	...to determine limits for how long I am in front of the TV, computer or video games.
	...to stop watching TV or using computer/video games in my leisure time to do physical activity.
	Confidence
	...to plan what I am going to watch on TV during the week.	Strongly Disagree; Disagree; Agree; Strongly Agree
	...to watch TV doing other activities that require body effort (clean, play) rather than sitting.
	...to plan what I am going to access on the computer or play video games during the week.
Expectations of screen time use	State how much you disagree or agree with the following statements:
	Positive expectations
	I think it is very relaxing when I sit in front of the TV.	Strongly Disagree; Disagree; Agree; Strongly Agree
	I feel good (happy) when I'm at the computer (talking or playing) or playing the video game.
	I get excited (agitated) when I'm using the computer or video game.
	Using the computer or video games is my way of connecting to the world (making friends).
	My friends would be sad if I shortened my time talking to them on the computer.
	I like to watch TV or use the computer/video games for many hours at a time.
	Watching TV or using the computer/video games is one of the things I enjoy doing in my leisure time.
	I watch TV or use the computer/video games to escape the world (obligations, discussions, problems).
	Negative expectations
	Watching TV or using the computer/video games traps me from doing important things (studying, eating).	Strongly Disagree; Disagree; Agree; Strongly Agree
	I get lazy after spending many hours in front of the TV, computer or video games.
	I feel pain in the body (back, legs) after spending many hours in front of the TV, computer or video games. Watching TV or using the computer/video games burns my eyes and leaves me with a headache.
Family model of screen time use	During a typical week, how often...
	TV use of household members
	...Do your parents (mother and/or father) watch TV?	Never; Sometimes; Often; Always
	...Do other people in your household watch TV?
	...Do you and your parents watch TV together?
	...Do you and others in your household watch TV together?
	Computer/games use of household members
	...Do your parents use the computer/video games?	Never; Sometimes; Often; Always
	...Do other people in your household use the computer/video games?
	...Do you and your parents use the computer/video games together?
	...Do you and others in your home use the computer/video games together?
Family rules of screen time use	During a typical week, how often do your parents...
	...limit the time that you can watch TV (e.g., only after dinner or when you finish your homework)?	Never; Sometimes; Often; Always
	...watch what you're watching on TV?
	...turn the TV off if they think you're watching TV for a long time?
	...limit the amount of time you can spend on the computer/video games (e.g., only after dinner or when you finish your homework)?
	...observe what you are accessing/playing on the computer or video game?
	...turn off computer/video games if they think you're playing for a long time?
Family beliefs of screen time use	In general, the people of my house…
	...think watching TV or using the computer/video games is a cool thing to do.	Strongly Disagree; Disagree; Agree; Strongly Agree
	...prefer to sit in front of the TV, computer or video games rather than doing physical activity.
	...think it's safer for me to stay at home in front of TV, computer or video games rather than doing physical activity outside the home.
Family support for reducing screen time use	In general, the people of my house…
	...encourage me to decrease the time watching TV or using the computer/video games.	Strongly Disagree; Disagree; Agree; Strongly Agree
	...comment to me that a lot of time in front of the TV or computer/video games can do harm to my health.
	...help me think about how I can decrease the time in front of the TV or computer/video games.
	...praise me when I spend less time in front of TV or computer/video games and more time doing physical activity.
	...prevent me from watching TV or using the computer/video games when I do something wrong.
School members' support for reducing screen time use	In general, the people of my school (colleagues, teachers or coordinators)
	...encourage me to decrease the time watching TV or using the computer/video games.	Strongly Disagree; Disagree; Agree; Strongly Agree
	...comment to me that a lot of time in front of the TV or computer/video games can do harm to my health.
	...help me think about how I can decrease the time in front of the TV or computer/video games.
	...praise me when I spend less time in front of the TV or computer/video games and more time doing physical activity.
Items of physical environmental variables	Number of TVs at home	Does not have; one; two; three or more
	Number of Video games at home
	Number of Computers with Internet access at home
	Number of Computers without Internet access at home
	Number of TVs in the bedroom
	Number of Video games in the bedroom
	Number of Computers with Internet access in the bedroom
	Number of Computers without Internet access in the bedroom