The next objective of the study was to determine the discriminant validity of psychosocial scales. Following item analysis, a principal components analysis (PCA) was used to determine the minimum number of components required to account for the pros, cons, self efficacy, control, subjective norms, descriptive norms, self identity, and susceptibility to disease items. Separate PCAs were performed on the 49 items in two randomly selected split half samples to examine the internal replicability of the solution, followed by a PCA on the 49 items in the full sample. Based on the results of these analyses, summated scales were developed and the reliability of each scale was assessed.
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Previous research suggests similar brief FFQs underestimate vegetable intake (see method section). Further evidence suggests the brief FFQ may be underestimating vegetable intake compared to the results of other N.Z. surveys (see discussion section).
The first PCA examined 49 items in a randomly selected split half sample. Bartlett’s test of sphericity, the Kaiser-Meyer-Olkin (KMO) statistic and individual measures of sampling adequacy supported the factorability of the correlation matrix. Bartlett’s test of sphericity was significant (χ² = 6071.98, p <.001), the KMO statistic was .82, and all individual measures of sampling adequacy were greater than .7065. Seven components were extracted for further analysis based on the results of the parallel analysis presented in Table 6.6 and the scree plot of eigenvalues attached in Appendix D. The component loadings, communalities, and percentage of variance accounted for by each component following varimax rotation are also attached in Appendix E. The rotated factor solution was approaching simple structure (Thurstone, 1947) with each component showing a number of strong loadings, and items loading onto only one component. The results of the second split half PCA yielded similar results and are also attached in Appendix E. The coefficient of congruence values for the split half analyses were all above .99 and therefore indicated a very high degree of similarity.
Table 6.6. Comparison of Eigenvalues obtained from the Principal Components Analysis (PCA) and Parallel Analysis in the First Split Half Sample, n = 230
Component number
Actual eigenvalue from PCA
Criterion value from parallel analysis Decision 1 8.448 2.035 Accept 2 6.344 1.917 Accept 3 4.164 1.839 Accept 4 2.873 1.766 Accept 5 2.372 1.707 Accept 6 2.008 1.641 Accept 7 1.680 1.598 Accept 8 1.209 1.535 Reject
While split half samples can be used to examine the internal replicability of results, Thompson (2004) recommends using the full sample results as the basis for interpretation as more confidence can be vested in results obtained from larger sample sizes. A PCA was therefore performed on all 49 items in the full sample.
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Bartlett’s test of sphericity should be significant p <.05 and the MSA for entire matrix and individual items should be at least .50. Tests of overall measures of correlation to justify application of factor analysis (Hair, Black, Babin, Anderson & Tatham, 2006).
Table 6.7. Inter-item Correlations, Means and Standard Deviations for Items Designed to Measure Psychosocial Factors, N = 458
Item Pros M SD 1 2 3 4 5 6 7 8 9
1 Give me vitamins and minerals 3.58 1.00 - 2 Feel good about looking after my health 3.46 1.06 0.70 - 3 Keep me regular (avoid constipation) 3.19 1.27 0.56 0.65 -
4 Help control weight 3.09 1.23 0.53 0.63 0.61 -
5 Help me look better 2.76 1.30 0.48 0.58 0.55 0.63 - 6 Feel good as F&Vs are fresh 3.13 1.16 0.50 0.60 0.59 0.58 0.67 - 7 Add variety to meals 3.15 1.10 0.49 0.55 0.64 0.55 0.55 0.68 - 8 Family would be pleased 2.54 1.32 0.42 0.49 0.46 0.44 0.45 0.48 0.49 - 9 Following food and nutrition guidelines 2.79 1.25 0.47 0.56 0.56 0.54 0.47 0.54 0.56 0.61 -
Cons 10 11 12 13 14 15 16 17 18
10 Other would think I was fussy 0.13 0.21 - 11 Meal planning would be difficult 0.14 0.21 0.46 -
12 Time consuming 0.13 0.20 0.35 0.49 -
13 Eating more would be expensive 0.20 0.24 0.28 0.29 0.49 - 14 Worry about pesticides 0.17 0.22 0.23 0.16 0.15 0.28 - 15 Feel I was overeating if I ate more 0.07 0.15 0.34 0.29 0.35 0.29 0.30 - 16 Give up other foods 0.11 0.19 0.27 0.32 0.34 0.33 0.21 0.46 - 17 Difficult to find F&Vs I like 0.12 0.20 0.21 0.34 0.37 0.31 0.19 0.31 0.35 - 18 Bad reaction 0.12 0.20 0.16 0.27 0.20 0.18 0.30 0.36 0.26 0.32 -
Self Efficacy 19 20 21 22 23 24 25 26 27
19 Can have for lunch 3.75 1.27 -
20 Can have when feeling tired 3.82 1.14 0.65 - 21 Can eat when favourite ones are unavailable 3.79 1.12 0.49 0.61 - 22 Can eat when in a rush 3.74 1.14 0.57 0.66 0.70 - 23 Can order when eating out 3.79 1.18 0.39 0.50 0.52 0.52 - 24 Can have when needs to be peeled and cut 3.77 1.16 0.56 0.47 0.60 0.60 0.48 - 25 Can have after fast food meal 3.13 1.47 0.39 0.45 0.46 0.53 0.47 0.53 - 26 Can include when preparing meal for myself 4.11 1.01 0.39 0.50 0.45 0.47 0.59 0.42 0.39 - 27 Can have small plate of vegetables for dinner 3.87 1.20 0.35 0.46 0.48 0.45 0.50 0.36 0.36 0.66 -
Item Norms M SD 28 29 30 31 32 33 34 28 People important to me think I should eat 5+ a day (agree) 4.26 1.85 -
29 Most of my family eat 5+ a day (agree) 3.72 1.46 0.58 - 30 Most of my friends eat 5+ a day (agree) 4.01 1.65 0.49 0.64 - 31 Most people important to me eat 5+ a day (agree) 4.22 1.75 0.61 0.77 0.69 - 32 People important to me expect me to eat 5+ a day (likely) 4.19 1.65 0.60 0.54 0.48 0.57 - 33 People important to me eat 5+ a day (true) 4.53 1.99 0.50 0.59 0.48 0.64 0.64 - 34 Other people like me eat 5+ a day (likely) 4.03 1.94 0.36 0.51 0.43 0.50 0.44 0.50 -
Susceptibility to Disease 35 36 37 38 39 40
35 High cholesterol level sometime in my life 3.00 1.13 - 36 Candidate for high blood pressure 2.82 1.16 0.50 - 37 Overweight some point in my life 2.67 1.24 0.40 0.42 - 38 Type of person who will get diabetes 2.20 0.98 0.30 0.40 0.44 - 39 Likely I’ll have a heart attack 2.71 0.99 0.46 0.46 0.35 0.41 - 40 I believe I will get cancer 2.71 0.91 0.23 0.21 0.21 0.28 0.41 -
Control 41 42 43 44 45 46
41 I am in control of the amount of F&Vs I eat 2.27 0.25 - 42 When I don't eat enough F&Vs, I am to blame 2.26 0.29 0.40 - 43 Other people determine the amount of F&Vs I eat 2.16 0.39 0.28 0.22 - 44 Not much I can do about the amount of F&Vs I eat 2.25 0.34 0.24 0.30 0.34 - 45 Whether or not I eat F&Vs each day is up to other people 2.30 0.30 0.24 0.26 0.49 0.49 - 46 Other people decide how much F&Vs I eat 2.28 0.31 0.31 0.29 0.61 0.31 0.66 -
Self Identity 47 48 49
47 I think of myself as a healthy eater 3.71 0.90 - 48 Concerned with healthy eating 3.74 0.89 0.56 - 49 Concerned with the health consequences of what I eat 3.78 0.89 0.42 0.76 -
Table 6.8. Component Loadings, Communalities (h²), and Percentage of Variance for Principal Components Extraction and Varimax Rotation
Component
1 2 3 4 5 6 7
h² after rotation
Feel good about looking after my health 0.81 0.70
Feel good as F&Vs are fresh 0.79 0.67
Keep me regular (avoid constipation) 0.78 0.65
Help control weight 0.77 0.64
Add variety to meals 0.76 0.63
Help me look better 0.75 0.61
Following food and nutrition guidelines 0.74 0.60
Give me vitamins and minerals 0.74 0.56
Family would be pleased 0.68 0.57
Can eat when in a rush 0.82 0.70
Can have when feeling tired 0.78 0.65
Can eat when favourite ones are unavailable 0.78 0.66
Can order when eating out 0.73 0.55
Can have when needs to be peeled and cut 0.72 0.57
Can have fruit for lunch 0.68 0.53
Can have after fast food meal 0.66 0.46
Can include when preparing meal for myself 0.65 0.53
Can have small plate of vegetables for dinner 0.64 0.49
Most people important to me eat 5+ a day (agree) 0.86 0.77
Most of my family eat 5+ a day (agree) 0.83 0.73
People important to me eat 5+ a day (true) 0.78 0.64
People important to me expect me to eat 5+ a day (likely) 0.78 0.63
Most of my friends eat 5+ a day (agree) 0.76 0.60
People important to me think I should eat 5+ a day (agree) 0.75 0.60
Component
1 2 3 4 5 6 7
h² after rotation
Feel I was overeating if I ate more 0.71 0.52
Time consuming 0.64 0.50
Give up other foods 0.64 0.46
Difficult to find F&Vs I like 0.59 0.40
Bad reaction 0.59 0.41
Meal planning would be difficult 0.59 0.44
Eating more would be expensive 0.55 0.40
Others would think I was fussy 0.50 0.40
Worry about pesticides 0.49 0.31
Other people decide how much F&Vs I eat 0.81 0.70
Whether or not I eat F&Vs each day is up to other people 0.81 0.68
Other people determine the amount of F&Vs I eat 0.78 0.63
Not much I can do about the amount of F&Vs I eat 0.67 0.54
When I don't eat enough F&Vs, I am to blame 0.58 0.43
I am in control of the amount of F&Vs I eat 0.52 0.50
Likely I'll have a heart attack 0.76 0.59
Candidate for high blood pressure 0.71 0.55
High cholesterol level sometime in my life 0.71 0.52
Overweight some point in my life 0.67 0.49
Type of person who will get diabetes 0.65 0.45
I believe I will get cancer 0.55 0.34
Concerned with healthy eating 0.81 0.76
Concerned with health consequences of what I eat 0.79 0.72
I think of myself as a healthy eater 0.62 0.60
The mean, standard deviation, and intercorrelations among items are presented in Table 6.7. Bartlett’s test of sphericity (χ² = 11106.41, p <.001), the KMO statistic (.87) and individual measures of sampling adequacy (>.70) supported the factorability of the correlation matrix. Seven components were extracted based on the scree plot of eigenvalues and parallel analysis (attached in Appendix F). The component loadings, communalities, and percentage of variance explained by each component following varimax rotation are summarised in Table 6.8. Component loadings under .40 have been suppressed to aid interpretation. In addition, all items have been ordered and grouped by size of component loading. The rotated factor solution is approaching simple structure (Thurstone, 1947) with each component showing a number of strong loadings, and items loading onto only one component. An examination of the communality values (proportion of variance in each item accounted for by the set of components) in Table 6.8 suggests the cons items are not as well represented in the factor solution.
Most items loaded onto their anticipated component. Components 1, 4, and 2 reflect the Pros, Cons and Self Efficacy respectively. Component 3 was interpreted as Perceived Norms given subjective and descriptive norm items loaded onto the same component. Component 5 was interpreted as Control as it contains internal and external control items. Components 6 and 7 reflect Susceptibility to Disease and Self Identity respectively. The total variance explained by the 7-component solution was 56.15%. Following rotation, the pros component explained the largest amount of variance (11.50%), followed by self efficacy (11.02%), perceived norms (9.07%), cons (7.14%), control (6.73%), perceived susceptibility to disease (6.12%), and self identity (4.58%).
The results of the PCA for the pros, cons, and self efficacy items were also compared to those of Ling (1999; Ling & Horwath, 1999, 2001) found during original scale development. There was a high degree of similarity in component loadings for the pros and cons items. The coefficient of congruence value was .99 for salient pros and cons items that were used in the current study and contained on Ling’s scales. However, in contrast to the results of Ling (1999; Ling & Horwath, 1999), the self efficacy items represented a single component following the deletion of three items contained on the original scale.
Summated scales were developed for the psychosocial factors based on the average score of salient items loading onto each component. The range, mean, standard deviation, and intercorrelations on each scale are summarised in Table 6.9. The intercorrelations among scales were small to moderate and therefore provide further evidence to suggest the psychosocial factors are relatively distinct constructs. Factor scores were also generated based directly upon component loadings. The relationship between factor scores and summated scale scores was very high with correlations ranging from .87 (self identity) to .98 (perceived norms). The reliability of each summated scale was also assessed using Cronbach’s alpha. As summarised in Table 6.9, each scale had an acceptable level of internal consistency with alpha coefficients ranging from .77 (susceptibility to disease) to .92 (pros).
Table 6.9. Intercorrelations, Means, Standard Deviations, and Cronbach’s Alpha Reliability Coefficients for the Psychosocial Summated Scales
Scale Range α M SD 1 2 3 4 5 6 1 Pros 1–5a .92 3.12 0.92 - 2 Cons‡ .0-.6a .80 0.14 0.13 .30 ** - 3 Self efficacy 1–5b .90 3.74 0.88 .15 ** -.22 ** - 4 Control† 1.4-2.5c .82 2.25 0.21 -.02 -.34 ** .28 ** - 5 Norms 1–7c .89 4.14 1.38 .19 ** -.10 * .24 ** .05 - 6 Self identity 1–5c .78 3.72 0.75 .21 ** -.18 ** .37 ** .20 ** .16 ** - 7 Susceptibility 1–5c .77 2.69 0.73 .07 .19 ** -.16 ** -.17 ** .04 -.26 **
† Log ‡ Square root. a 1 = not important – 5 = extremely important. b 1 = not at all confident – 5 = totally confident. c 1 = strongly disagree – 7 = strongly agree.