Exploring the Dunning-Kruger Effect in Health Communication: How Perceived Food and Media Literacy and Actual Knowledge Drift Apart When Evaluating Misleading Food Advertising

Figures & data

Table 1. Means and standard deviations for each group’s actual ability (=food and media knowledge) and self-assessment (=food and media literacy)

	“lowest knowledge”	“low knowledge”	“high knowledge”	“highest knowledge”
	(n = 232)	(n = 139)	(n = 301)	(n = 328)
Self-assessment (food & media literacy)	84.77 (21.09)	85.83 (19.79)	88.72 (18.95)	92.42 (17.68)
Actual ability (food & media knowledge)	7.15 (1.01)	9.00 (.00)	10.46 (.50)	13.10 (1.21)

For self-assessment respondents could reach a maximum score of 133 points (highest possible sum score of the food and media literacy scales). 21 points could be reached in total for actual ability (scores on the objective knowledge test on food and media knowledge). For both, self-assessment and actual ability, higher scores indicated more of the construct. Respondents reaching zero to eight points were grouped into “lowest knowledge” (n = 232), nine points into “low knowledge” (n = 139), ten to eleven points into “high knowledge” (n = 301), and twelve and more points into “highest knowledge” (n = 328).

Figure 1. Differences between actual ability (food & media knowledge) and self-assessment (food & media literacy) per quartile.

Table 2. Means and standard deviations across groups

Variables/Groups	Mean (SD)
Opinion leader persuasiveness
Lowest k.	3.56 (1.15)
Low	3.28 (1.16)
High	3.37 (1.16)
Highest	3.50 (1.12)
Opinion leader mavenness
Lowest k.	3.93 (1.37)
Low	3.94 (1.25)
High	4.09 (1.34)
Highest	4.19 (1.24)
Opinion leader connectivity
Lowest k.	3.56 (1.34)
Low	3.28 (1.42)
High	3.37 (1.40)
Highest	3.50 (1.41)
Passive media usage
Lowest k.	3.43 (1.64)
Low	3.26 (1.59)
High	3.35 (1.58)
Highest	3.48 (1.63)
Active media usage
Lowest k.	2.86 (1.59)
Low	2.67 (1.53)
High	2.58 (1.51)
Highest	2.60 (1.47)
Skepticism toward advertising
Lowest k.	4.47 (1.42)
Low	4.68 (1.36)
High	4.66 (1.33)
Highest	4.89 (1.32)
Purchase intention
Lowest k.	3.38 (1.74)
Low	3.18 (1.76)
High	3.17 (1.75)
Highest	2.89 (1.58)
Cognitive involvement with a healthy diet
Lowest k.	4.88 (1.54)
Low	5.29 (1.26)
High	5.42 (1.19)
Highest	5.43 (1.19)
Ad evaluation (ad liking)
Lowest k.	4.38 (1.67)
Low	4.57 (1.64)
High	4.43 (1.65)
Highest	4.34 (1.56)

Table A1. Sociodemographic characteristics of participants

Characteristic	n	%
Gender
Female	511	51.1
Male	487	48.7
Diverse	2	0.2
Age group
16–30	219	21.9
31–40	158	15.8
41–50	176	17.6
51–60	178	17.8
61–70	128	12.8
≥71	141	14.1
Federal state
Vienna	213	21.3
Lower Austria	160	19.0
Burgenland	34	3.4
Styria	143	14.3
Carinthia	65	6.5
Upper Austria	165	16.5
Salzburg	62	6.2
Tyrol	85	8.5
Vorarlberg	43	4.3
Highest completed level of education
Compulsory school	85	8.5
Vocational school	476	47.6
Secondary school without school leaving examination	128	12.8
School leaving examination (Matura)	181	18.1
Completed course of studies at a university or university of applied sciences	130	13.0
Employment
Self-employed, freelance, executive employees	62	6.2
Public servants, employees	321	32.1
Worker	141	14.1
Farmer	2	0.2
Pupil, student	66	6.6
Not employed (work in household, parental leave, unemployed)	137	13.7
Retired	271	27.1

Table A2. Applied measures – items with means, standard deviations; reliability estimates: confirmatory factor analyses results, cronbach alpha

Variables	Items	M	SD	α
Food literacy (self-assessment) (based on Short food literacy questionnaire – SFLQ) (Krause, Beer-Borst, Sommerhalder, Hayoz, & Abel, 2018)^1	General functional food literacy			.86
	In general, how well do you understand the following types of nutritional information?
	(A) Nutrition information leaflets^a	4.11	2.29
	(B) Information about nutrition in newspapers, magazines, etc^.a	4.22	2.05
	(C) Information about nutrition on the Internet^a	4.61	2.04
	(D) TV or radio program on nutrition^a	4.12	2.14
	(E) Oral recommendations regarding nutrition from professionals^a	4.83	2.10
	(F) Nutrition advice from family members or friends^a	4.50	1.98
	Critical food literacy			.87
	How easy is it for you to judge if media information on nutritional issues can be trusted?^b	4.45	1.37
	Commercials often relate foods with health. How easy is it for you to judge if the presented associations are appropriate or not?^b	4.51	1.49
	How easy is it for you to evaluate if a specific food is relevant for a healthy diet?^b	4.89	1.34
	How easy is it for you to evaluate the longer-term impact of your dietary habits on your health?^b	4.99	1.37
	Functional food literacy in the Austrian context			.85
	How familiar are you with the Austrian Food Pyramid?^c	4.13	1.97
	I know the official Austrian recommendations about fruit and vegetable consumption^.c	4.54	1.74
	I know the official Austrian recommendations about salt intake.^c	4.11	1.74
	CFA: χ^2(62) = 417.7, p .000, CFI = 0.95, TLI = 0.93, RMSEA = 0.08, SRMR = 0.04
Media literacy (self-assessment) (Pinkleton & Austin, 2016)	Perceived critical thinking skills regarding message content			.75
	It is important to think twice about what messages say.	5.41	1.49
	I look for more information before I believe something I see in messages.	5.39	1.33
	I think about things I see before I accept them as believable.	5.64	1.18
	Perceived critical thinking skills regarding message source			.90
	I think about how someone created a message.	4.70	1.57
	I think about what the creator of a message wants me to think.	4.87	1.53
	I think about why someone created a message I see.	4.59	1.64
	CFA: χ^2(8) = 52.5, p .000, CFI = 0.99, TLI = 0.97, RMSEA = 0.08, SRMR = 0.03
Food knowledge (actual ability) (based on actual product information of an existing cereal bar as criticized by Foodwatch)^2	What’s your guess? 100 grams of this product consist of: (Choose the right answer)
	9%/19%/29% of fiber
	5%/15%/25% of protein
	3%/13%/23% of fat
	0,27%/0,37%/0,47% of salt
	4%/14%/24% of sugar
	20%/30%/40% of carbohydrates
Food knowledge (actual ability) (based on Parmenter & Wardle, 1999)	Do you think this product is either high or low in the following ingredients? (Chose the right answer)
	-fiber (high/low)
	-protein (high/low)
	-fat (high/low)
	-salt (high/low)
	-sugar (high/low)
	-carbohydrates (high/low)
Media knowledge (actual ability) (based on critique by Foodwatch’s food experts^2 and a team of four experts from the media and health communication field)	In this ad you can see different graphical elements and texts. Some of these elements in this ad aim at conveying healthiness. Please click on all those elements from the list below, where you think that they try to convey healthiness.
	-Chocolate combined with cereals as a high-quality snack
	-The shape of the female silhouette
	-The novelty of the product on the market
	-UTZ certified cacao
	-The enjoyment of chocolate with a high cocoa content
	-The light color of the ad
	-Protein/fewer carbohydrates
	-The name of the producer (Korni)
	-The prominent illustration of cereals high in fiber
Active media usage (Adapted from Li, 2016; Pagani & Mirabello, 2011)	I comment on others’ posts on social media sites.^d	2.39	1.57	.92
	I “like” posts on social media sites.^d+e	3.11	1.86
	I share contents on social media sites with my connections.^d	2.66	1.65
	I post contents on my own social media page.^d	2.51	1.65
Passive media usage (Adapted from Li, 2016; Pagani & Mirabello, 2011)	I watch videos or pictures posted on social media sites.^d	3.73	1.80	.89
	I read online discussions on social media sites.^d	3.05	1.74
	I read user comments/ratings/reviews on social media sites.^d	3.41	1.77
	CFA: χ^2(13) = 194.5, p .000, CFI = 0.97, TLI = 0.95, RMSEA = 0.12, SRMR = 0.03
Opinion leadership “Connectivity, Persuasiveness, and Maven Scales”^3 (Boster, Kotowski, Andrews, & Serota, 2011)	Opinion leader connectivity			.92
	I’m often the link between friends in different groups.	3.33	1.63
	I often find myself introducing people to each other.	3.48	1.56
	I try to bring people I know together when I think they would find each other interesting.	3.61	1.66
	I frequently find that I am the connection between people who would not otherwise know one another.	3.50	1.63
	The people I know often know each other because of me.	3.31	1.56
	Opinion leader persuasiveness			.91
	I am good at thinking of multiple ways to explain my position on an issue.	4.46	1.48
	When in a discussion, I’m able to make others see my side of the issue.	4.50	1.31
	I am able to adapt my method of argument to persuade someone.	4.52	1.31
	I can effortlessly offer multiple perspectives on an issue which support my position.	4.52	1.35
	More often than not, I am able to convince others of my position during an argument.	4.49	1.25
	Opinion leader mavenness			.89
	When I know something about a healthy lifestyle topic, I feel it is important to share that information with others.	4.36	1.50
	I like to be aware of the most up-to-date healthy lifestyle information so I can help others by sharing when it is relevant.	4.13	1.62
	If someone asked me about a healthy lifestyle issue that I was unsure of, I would know how to help them find the answer.	4.47	1.52
	Being knowledgeable enough about healthy lifestyles so that I could teach someone else is important to me.	4.15	1.59
	People often seek me out for answers when they have questions about a healthy lifestyle issue.	3.23	1.62
	CFA: χ^2(87) = 464.7, p.000, CFI = 0.96, TLI = 0.96, RMSEA = 0.07, SRMR = 0.05
Skepticism toward advertising in general (Koinig et al., 2018; Obermiller & Spangenberg, 1998)	I can depend on getting the truth in most advertising.*	5.11	1.51	.84
	Advertising’s aim is to inform the consumer.*	4.24	1.67
	I feel I have been accurately informed after viewing most advertisements.*	4.72	1.50
Purchase intention (Bredahl, 2001)	If this product was available in the shops, I would intend to definitely buy it.	3.23	1.76	.95
	I am strongly for buying this product.	3.03	1.73
	CFA: χ^2(4) = 7.95, p .093, CFI = 1.00, TLI = 1.00, RMSEA = 0.03, SRMR = 0.01
Ad evaluation (ad liking)^4 (Diehl, Terlutter, & Mueller, 2016; MacKenzie & Lutz, 1989)	The ad is likable.	4.41	1.62
Cognitive involvement with a healthy diet (Schaefer, Terlutter, & Diehl, 2019; Zaichkowsky, 1985, 1994)	A healthy diet is important to me.^f	5.28	1.31

Only retained items for the analyses are reported here. *marked items were reverse coded.

^aseven-point Likert scale from “very bad” to “very good” with one additional response option “I do not make use of this kind of information” – coded as zero. ^bseven-point Likert scale from “very hard” to “very easy”. ^cseven-point Likert scale from “very bad” to “very good” with one additional response option “I do not know it” – coded as zero. ^dseven-point Likert scale from “never” to “always”. ^eThis item’s wording has been changed as the original item related very specifically to companies’ Facebook sites (I “like” posts on companies’ Facebook sites – clicking the like button) (Li, 2016). ^fWe used this item to measure cognitive involvement with a healthy diet as it has previously been identified to measure cognitive involvement (Zaichkowsky, 1994). The selection of the one item was based on highest loadings received in Schaefer et al. (2019).

¹The SFLQ is based on a research project that was previously conducted in Switzerland. Hence, German items were already available from this research (Beer-Borst, 2017).

²Foodwatch (n.d.)

³We used the “Connectivity, Persuasiveness, and Maven Scales” (Boster et al., 2011) to measure Opinion leadership. These are three subscales which aim at detecting “3 types of influential others” (Boster et al., 2011, p. 178). The “Connectivity” scale aims at identifying those people who are well-connected and who bring people together. The “Persuasiveness” scale identifies those people who are good at, for example, convincing others. The “Maven” scale aims at identifying the so-called (health) mavens who are also regarded as health experts and are often consulted by friends regarding health topics (Boster et al., 2011, p. 192). The three subscales were also reflected in our data, in line with the original research paper (Boster et al., 2011).

⁴Perceived desirability captures “individuals” positive affect toward media portrayals, typically measured as expressions of message liking or enjoyment of how reference groups are portrayed in advertising or programming” (Austin, Pinkleton, Chen, & Austin, 2015, p. 403). Since our aim is to evaluate reactions toward a misleading food ad which does not display a reference group of participants, we chose to measure ad evaluation via “ad liking”, which has also been previously used to measure people’s evaluation of ads (Diehl et al., 2016) and affective reactions toward ads (Schaefer, Terlutter, & Diehl, 2020).

Table A3. Objective knowledge test with correct answers (marked in bold)

	What’s your guess? 100 grams of this product consist of:
1.	9%	19%	29%	of fiber
2.	5%	15%	25%	of protein
3.	3%	13%	23%	of fat
4.	0,27%	0,37%	0,47%	of salt
5.	4%	14%	24%	of sugar
6.	20%	30%	40%	of carbohydrates
	Do you think this product is either high or low in the following ingredients?
7.	fiber		High	low
8.	protein		High	low
9.	fat		High	low
10.	salt		High	low
11.	sugar		High	low
12.	carbohydrates		High	low
	In this ad you can see different graphical elements and texts. Some of these elements in this ad aim at conveying healthiness. Please click on all those elements from the list below, where you think that they try to convey healthiness.
13.	Chocolate combined with cereals as a high-quality snack			X
14.	The shape of the female silhouette			X
15.	The novelty of the product on the market
16.	UTZ certified cacao
17.	The enjoyment of chocolate with a high cocoa content			X
18.	The light color of the ad			X
19.	Protein/fewer carbohydrates			X
20.	The name of the producer (Korni)			X
21.	The prominent illustration of cereals high in fiber			X

Twelve questions (1–12) measured food knowledge (actual ability), nine questions (13–21) measured media knowledge (actual ability). For each correct answer participants could reach one point while zero points were given for incorrect answers. A total score of twenty-one points could be achieved. For actual food knowledge: The use of these six ingredients (questions 1–6) was based on the actual product information of an existing cereal bar that has been criticized by Foodwatch (n.d.). Correct answers (questions 7–12) were based on findings provided by research (Schulze-Lohmann, 2012), the German Association for Nutrition (Deutsche Gesellschaft für Ernährung, n.d.) and on the critique by Foodwatch which classified certain amounts of the product’s ingredients as either high or low. For media knowledge (actual ability): A team of four experts from the media and health communication field defined which response options transport healthiness and which ones don’t. Correct answers were also based on the critique published by Foodwatch’s food experts. Prior research followed a similar procedure: Lyons, Montgomery, Guess, Nyhan, and Reifler (2021) also consulted experts (e.g., fact-checking organization) to ensure the correctness of answers in the knowledge test. In our research, we also used scientific findings and the expertise of Foodwatch and experts from the media and health communication field to ensure the correctness and reliability of our test and answers.

Our objective knowledge test was based on an existing (misleading) food ad. Similarly, prior research (e.g., by Lyons et al., 2021) also used real-world examples (news articles) as basis to measure people’s actual ability to detect false news content.

In line with most prior studies on the DKE (Aqueveque, 2018; McMahon et al., 2020; Sullivan et al., 2019), we used multiple-choice and yes/no response options in our objective knowledge test. Here, research has also started to use different scales to measure actual ability, which might be an alternative for future studies as well (see Lyons et al., 2021).

Table A4. Results of the Post-hoc tests: for all four groups

				95% CI
Variables/Groups			Mean difference	LL	UL	p
Opinion leader persuasiveness
	Lowest k.	Low k.	.034	−.2069	.2751	.781
		High	−.106	−.3019	.0907	.292
		Highest	−.276*	−.4690	−.0834	.005*
	Low	Lowest	−.034	−.2751	.2069	.781
		High	−.140	−.3700	.0906	.234
		Highest	−.310*	−.5377	−.0830	.008*
	High	Lowest	.106	−.0907	.3019	.292
		Low	.140	−.0906	.3700	.234
		Highest	−.171	−.3499	.0086	.062
	Highest	Lowest	.276*	.0834	.4690	.005*
		Low	.310*	.0830	.5377	.008*
		High	.171	−.0086	.3499	.062
Opinion leader mavenness
	Lowest k.	Low k.	−.009	−.2838	.2654	.947
		High	−.160	−.3835	.0638	.161
		Highest	−.254*	−.4741	−.0348	.023*
	Low	Lowest	.009	−.2654	.2838	.947
		High	−.151	−.4130	.1118	.260
		Highest	−.245	−.5042	.0138	.063
	High	Lowest	.160	−.0638	.3835	.161
		Low	.151	−.1118	.4130	.260
		Highest	−.095	−.2988	.1096	.364
	Highest	Lowest	.254*	.0348	.4741	.023*
		Low	.245	−.0138	.5042	.063
		High	.095	−.1096	.2988	.364
Opinion leader connectivity
	Lowest k.	Low k.	.283	−.0103	.5766	.059
		High	.194	−.0455	.4325	.112
		Highest	.065	−.1697	.2998	.587
	Low	Lowest	−.283	−.5766	.0103	.059
		High	−.090	−.3700	.1907	.531
		Highest	−.218	−.4949	.0586	.122
	High	Lowest	−.194	−.4325	.0455	.112
		Low	.090	−.1907	.3700	.531
		Highest	−.129	−.3467	.0897	.248
	Highest	Lowest	−.065	−.2998	.1697	.587
		Low	.218	−.0586	.4949	.122
		High	.12849	−.0897	.3467	.248
Skepticism toward advertising
	Lowest k.	Low k.	−.213	−.4976	.0726	.144
		High	−.191	−.4230	.0415	.107
		Highest	−.420*	−.6480	−.1918	<.001**
	Low	Lowest	.213	−.0726	.4976	.144
		High	.022	−.2507	.2942	.875
		Highest	−.207	−.4764	.0615	.131
	High	Lowest	.191	−.0415	.4230	.107
		Low	−.022	−.2942	.2507	.875
		Highest	−.229*	−.4412	−.0171	.034*
	Highest	Lowest	.420**	.1918	.6480	<.001**
		Low	.207	−.0615	.4764	.131
		High	.229*	.0171	.4412	.034*
Purchase intention
	Lowest k.	Low k.	.202	−.1558	.5588	.269
		High	.218	−.0735	.5085	.143
		Highest	.493**	.2073	.7790	<.001**
	Low	Lowest	−.202	−.5588	.1558	.269
		High	.016	−.3254	.3574	.927
		Highest	.292	−.0454	.6286	.090
	High	Lowest	−.218	−.5085	.0735	.143
		Low	−.016	−.3574	.3254	.927
		Highest	.276*	.0099	.5414	.042*
	Highest	Lowest	−.493**	−.7790	−.2073	<.001**
		Low	−.292	−.6286	.0454	.090
		High	−.276*	−.5414	−.0099	.042*
Cognitive involvement with a healthy diet
	Lowest k.	Low k.	−.413*	−.68	−.14	.003*
		High	−.539**	−.76	−.32	<.001**
		Highest	−.547**	−.76	−.33	<.001**
	Low	Lowest	.413*	.14	.68	.003*
		High	−.127	−.39	.13	.339
		Highest	−.134	−.39	.12	.304
	High	Lowest	.539**	.32	.76	<.001**
		Low	.127	−.13	.39	.339
		Highest	−.008	−.21	.19	.941
	Highest	Lowest	.547**	.33	.76	<.001**
		Low	.134	−.12	.39	.304
		High	.008	−.19	.21	.941
Passive media usage
	Lowest k.	Low k.	.172	−.1666	.5113	.319
		High	.081	−.1950	.3572	.564
		Highest	−.046	−.3170	.2254	.740
	Low	Lowest	−.172	−.5113	.1666	.319
		High	−.091	−.4151	.2327	.581
		Highest	−.218	−.5379	.1016	.181
	High	Lowest	−.081	−.3572	.1950	.564
		Low	.091	−.2327	.4151	.581
		Highest	−.127	−.3790	.1252	.323
	Highest	Lowest	.046	−.2254	.3170	.740
		Low	.218	−.1016	.5379	.181
		High	.127	−.1252	.3790	.323
Active media usage
	Lowest k.	Low k.	.192	−.1279	.5121	.239
		High	.285*	.0239	.5451	.032*
		Highest	.262*	.0061	.5181	.045*
	Low	Lowest	−.192	−.5121	.1279	.239
		High	.092	−.2134	.3982	.553
		Highest	.070	−.2318	.3718	.649
	High	Lowest	−.285*	−.5451	−.0239	.032*
		Low	−.09239	−.3982	.2134	.553
		Highest	−.02237	−.2603	.2156	.854
	Highest	Lowest	−.26212*	−.5181	−.0061	.045*
		Low	−.07002	−.3718	.2318	.649
		High	.02237	−.2156	.2603	.854
Ad evaluation (ad liking)
	Lowest k.	Low k.	−.191	−.53	.15	.272
		High	−.053	−.33	.23	.710
		Highest	.043	−.23	.32	.760
	Low	Lowest	.191	−.15	.53	.272
		High	.139	−.19	.47	.405
		Highest	.234	−.09	.56	.155
	High	Lowest	.053	−.23	.33	.710
		Low	−.139	−.47	.19	.405
		Highest	.095	−.16	.35	.462
	Highest	Lowest	−.043	−.32	.23	.760
		Low	−.234	−.56	.09	.155
		High	−.095	−.35	.16	.462

LSD Post-hoc tests were interpreted. CI = confidence interval. LL = lower limit. UL = upper limit. *p < .05 **p < .001. Significant differences are marked in bold.

Table A5. Descriptive statistics and correlations for study variables

Variable	M	SD	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
1. General functional FL	4.40	1.60	-
2. Critical FL	4.71	1.19	.47**	-
3. Functional FL (Austria)	4.26	1.60	.47**	.50**	-
4. ML (Content)	5.48	1.10	.35**	.41**	.24**	-
5. ML (Source)	4.72	1.45	.25**	.31**	.17**	.55**	-
6. Food Knowledge	6.13	1.79	.05	.02	.03	−.01	.01	-
7. Media Knowledge	4.23	1.53	.17**	.17**	.08	.19**	.13**	.07*	-
8. Active MU	2.67	1.52	.14**	.17**	.13**	.07*	.15**	−.03	−.04	-
9. Passive MU	3.40	1.61	.22**	.22**	.15**	.20**	.21**	−.01	.05	.76**	-
10. OL (C)	3.45	1.39	.21**	.20**	.23**	.14**	.24**	.00	.03	.35**	.28**	-
11. OL (P)	4.50	1.15	.29**	.33**	.22**	.38**	.37**	.04	.12**	.23**	.26**	.48**	-
12. OL (M)	4.07	1.31	.43**	.41**	.45**	.32**	.37**	.05	.08*	.30**	.32**	.47**	.51**	-
13. SKEP	4.70	1.36	−.18**	−.05	−.12**	.10**	−.01	.08**	.09**	−.27**	−.22**	−.22**	−.10**	−.27**	-
14. PI	3.13	1.70	.13**	.02	.05	−.10**	.05	−.10**	−.03	.30**	.30**	.24**	.07*	.26**	−.52**	-
15. Ad eval. (liking)	4.41	1.62	.20**	.04	.08*	.02	−.02	−.06	.06	.16**	.21**	.10**	.08*	.20**	−.42**	.62**	-
16. INV	5.28	1.31	.41**	.40**	.39**	.37**	.26**	.08**	.15**	.01	.08**	.16**	.23**	.53**	−.05	.08**	.13**	-

Pearson’s r correlations. n = 1,000 for all variables. *p < .05 **p < .01. FL = Food Literacy, ML = Media Literacy, MU = Media Usage, OL = Opinion Leadership, C = Connectivity, P = Persuasiveness, M = Mavenness, SKEP = Skepticism toward advertising in general, PI = Purchase Intention, Ad eval. = Ad evaluation/ad liking, INV = Cognitive Involvement with a healthy diet.

Table A6. Number of participants per group, grouping of participants based on scores on objective test, means and standard deviations for each group’s: actual ability (=food knowledge) and self-assessment (=food literacy); actual ability (media knowledge) and self-assessment (=media literacy)

	“lowest knowledge”	“low knowledge”	“high knowledge”	“highest knowledge”
	(n = 181) 0–4 points	(n = 218) 5 points	(n = 203) 6 points	(n = 398) 7–12 points
self-assessment (food literacy)	57.61 (16.99)	58.22 (16.13)	55.62 (16.25)	59.32 (14.49)
actual ability (food knowledge)	3.64 (.62)	5.00 (.00)	6.00 (.00)	7.94 (1.02)
	“lowest knowledge”	“low knowledge”	“high knowledge”	“highest knowledge”
	(n = 105) 0–2 points	(n = 233) 3 points	(n = 238) 4 points	(n = 424) 5–9 points
self-assessment (media literacy)	28.84 (6.51)	29.77 (7.29)	29.89 (6.28)	31.89 (6.56)
actual ability (media knowledge)	1.61 (.52)	3.00 (.00)	4.00 (0.00)	5.68 (.89)

For self-assessment respondents could reach a maximum sum score of 1) 91 points for food literacy − 13 items measuring food literacy – highest response option “7” 2) 42 points for media literacy − 6 items measuring media literacy – highest response option “7.” For actual ability, a maximum score of 1) 12 points could be reached for food knowledge 2) 9 points for media knowledge in total (scores on the objective knowledge test). For both, self-assessment and ability, higher scores indicated more of the construct.

Figure A1. Differences between 1) actual ability (food knowledge) and self-assessment (food literacy) 2) actual ability (media knowledge) and self-assessment (media literacy) per quartile.

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Data availability statement

Data available on request from the authors.