Development and application of a reliable and valid measurement tool
4.2 Part 1: Development and validation of the instrument
4.2.3.2 Internal consistency and test-retest reliability
Reliability and consistency analyses are presented in Table 4.2. Internal consistency was
acceptable for each of the four scales (Cronbach’s alpha 0.55 − 0.75) (Cortina, 1993), particularly given their small number of items and their intended purpose to compare between groups rather than within individuals. Test-retest reliability was good for all attitude scales (r>0.80, p<0.01) (Salkind, 2006) and was acceptable (r>0.60, p<0.001) for over half of the individual attitude items.
Overall, the MMR individual items and scale were the strongest elements of the questionnaire.
126 4.2.3.3 Concurrent validity (univariate predictors of MMR receipt)
Concurrent validity analyses for the attitudes element of the instrument are presented in Table 4.3.
Concurrent validity was demonstrated for over half of the individual attitude items and three of the five attitude scales. Eleven individual items predicted both MMR dose 1 and MMR dose 2 receipt (p<0.05), many of which were MMR attitudes and information source attitudes, and none of which were measles attitudes. Chief among them in terms of effect size were preference for single
measles, mumps and rubella vaccines (partial Eta squared = 0.06-0.09, thus 6-9% of variance in MMR receipt explained), previous MMR acceptance/rejection (5-9% variance explained), and wishing to protect others through vaccinating one’s own child (6-8% variance explained). The MMR attitudes, social attitudes and information attitudes scales all predicted both MMR dose 1 and MMR dose 2 receipt (p<0.001) with small to medium effects (4-10% variance explained by each scale; Stevens, 2002). The measles scale predicted only dose 1 receipt (p<0.05) but the effect size was very small (1% variance explained) and no pairwise comparisons were significant. Adjustment for child age and deprivation generally had minimal effects on differences between MMR status groups, and the items on which adjustment effects were observed were more strongly related to MMR status among parents of younger children, who were concentrated in the late MMR dose 1 and on-time MMR dose 2 groups . Generally parents who gave their child MMR dose 1 on time differed from parents who gave no MMR but not from parents who gave MMR late.
Concurrent validity analyses for the demographics element of the instrument are presented in Table 4.4. Only parent ethnicity predicted both MMR dose 1 and MMR dose 2 receipt, with white British parents less likely than parents in other groups to give their children either dose on time. This effect persisted when child age and deprivation were controlled for (p<0.001–p<0.05). Children aged 5-6 were significantly more likely to have received MMR dose 1 late than on-time or not at all (p<0.001), but MMR status did not vary by child age for older children or for MMR dose 2. On-time receipt of
127 MMR dose 2 was associated with smaller family size (p<0.05) and married parents, though the latter relationship appeared to be a function of deprivation (adjusted p>0.05), as married parents were less likely than other parents to be deprived (p<0.001).
128 Table 4.2: Internal consistency and test-retest reliability
Scale (n) Item (n)
Test-retest: Pearson’s r
(p-level)a Scale internal consistency
Item Scale Cronbach’s
alpha N missing data
MMR
Attitudes (411)
MMR has serious side effects† (518) 0.76 (<0.001)
0.89 (<0.01) 0.75
56 missing 1 17 missing 2 11 missing 3+
MMR will protect my child against measles (519) 0.30 (<0.05) I have seen or heard about bad reactions to MMR† (521) 0.67 (<0.001) MMR is too much for my child’s body to cope with† (519) 0.80 (<0.001) I‘d prefer to give single measles, mumps and rubella shots† (516) 0.90 (<0.001) I have not given my other children MMR† (452) 0.76 (<0.001) I would feel very bad if my child had a reaction to MMR† (512) 0.64 (<0.001)
Measles Attitudes (488)
Measles is a serious illness (518) 0.38 (<0.01)
0.86 (<0.01) 0.63
49 missing 1 10 missing 2 13 missing 3+
I have seen or heard about bad cases of measles (515) 0.59 (<0.001) Without MMR, it is likely that my child will catch measles (521) 0.65 (<0.001) It is better to get natural immunity by catching measles† (516) 0.75 (<0.001) I would feel very bad if my child caught measles (521) 0.45 (<0.001) Social attitudes
(505)
I’d like to protect other children by giving my child MMR (516) 0.68 (<0.001)
0.80 (<0.01) 0.68 - People important to me think I should give my child MMR (514) 0.58 (<0.001)
Information attitudes (486)
I trust my GP and nurse to advise me about MMR (523) 0.53 (<0.001)
0.80 (<0.01) 0.55
36 missing 1 11 missing 2 15 missing 3+
Not enough scientific research has been done on MMR† (519) 0.66 (<0.001) I have done my own research about MMR (e.g. online)† (508) 0.72 (<0.001) I do not believe newspaper and TV stories about MMR (518) 0.35 (<0.01) I started thinking about MMR before my child was born† (514) 0.55 (<0.001) Practicalities
(518)
Time/transport/cost makes it hard to take my child for MMR†
(518) 0.49 (<0.001) - - -
†: Reverse scored. a: Test-retest n=79
129 Table 4.3: Concurrent validity (univariate analyses of attitudes items and scales)
MMR dose 1 MMR dose 2 MMR will protect my child
against measles 4.1 (0.9)b 4.1 (0.7)b 4.0 (0.9)b 0.16 0.13 0.01 4.1 (0.9) 4.0 (0.9) 0.16 0.18 0.003 I have seen or heard about
bad reactions to MMR† 2.7 (1.1)b 2.3 (1.1)bc 2.3 (1.2)c <0.01 <0.01 0.02 2.7 (1.1) 2.3 (1.1) <0.001 <0.001 0.03 I have seen or heard about
bad cases of measles 3.6 (1.1)b 3.6 (1.1)b 3.7 (1.0)b 0.58 0.70 0.001 3.5 (1.1) 3.6 (1.1) 0.26 0.21 0.003 MMR is too much for my
child’s body to cope with† 3.6 (1.1) 3.1 (1.1)b 3.0 (1.2)b <0.001 <0.001 0.01 3.6 (1.1) 3.2 (1.2) <0.001 <0.001 0.03 I‘d prefer to give single
measles, mumps and rubella shots†
3.2 (1.1) 2.5 (1.2)b 2.4 (1.3)b <0.001 <0.001 0.09 3.2 (1.1) 2.6 (1.3) <0.001 <0.001 0.06 Without MMR, it is likely that
my child will catch measles 3.6 (1.0)b 3.3 (1.0)b 3.3 (1.0)b <0.05 0.06 0.01 3.6 (1.0) 3.4 (1.0) 0.06 0.05 0.01 It is better to get natural
immunity by catching
130 children by giving my child
MMR
I have done my own research
about MMR (e.g. online)† 3.0 (1.2)b 2.6 (1.2)bc 2.6 (1.1)c <0.001 <0.001 0.03 3.1 (1.2) 2.7 (1.2) <0.001 <0.001 0.03 I have not given my other
children MMR† 4.2 (1.0) 3.5 (1.3)b 3.3 (1.4)b <0.001 <0.001 0.09 4.2 (1.0) 3.6 (1.3) <0.001 <0.001 0.05 I do not believe newspaper
and TV stories about MMR 3.2 (0.9)b 3.1 (0.8)b 3.0 (0.9)b 0.06 0.06 0.01 3.1 (0.9) 3.1 (0.9) 0.60 0.58 0.001 I would feel very bad if my
child caught measles 4.3 (1.0)b 4.1 (1.0)b 4.1 (1.0)b 0.12 0.14 0.01 4.4 (0.9) 4.1 (1.0) <0.01 <0.01 0.01 I would feel very bad if my
child had a reaction to MMR† 1.8 (0.9)b 1.7 (0.9)bc 1.5 (0.8)c <0.05 <0.05 0.01 1.8 (1.0) 1.6 (0.8) 0.07 <0.05 0.01 I started thinking about MMR
before my child was born† 3.3 (1.3)b 3.5 (1.3)b 2.9 (1.3) <0.001 <0.001 0.04 3.3 (1.3) 3.1 (1.3) 0.26 0.23 0.03 Time/transport/cost makes it
hard to take my child for MMR†
4.3 (0.8)b 4.2 (0.7)b 4.3 (0.8)b 0.80 0.47 0.003 4.3 (0.9) 4.3 (0.8) 0.86 0.72 0.000 People important to me think
I should give my child MMR 3.5 (1.1)b 3.2 (1.2)bc 3.0 (1.2)c <0.001 <0.001 0.05 3.6 (1.0) 3.1 (1.2) <0.001 <0.001 0.04
†: reverse scored items; for all items higher scores = more ‘pro-MMR’ attitude. a: Adjusted for child age and IMD2007 score. Values which do not differ from one another are marked with the same superscript letter. d: Adjusted effect size = partial Eta squared.
131 Table 4.4: Concurrent validity (univariate analyses of demographics items)
MMR dose 1 MMR dose 2
132
MMR dose 1 MMR dose 2
n (%) Comparison n (%) Comparison
On-time Late None Unadj Adj a On-time None Unadj Adj a
2 111 (47) 34 (52) 100 (47) 73 (46) 172 (49)
3 62 (27) 16 (24) 45 (21) 45 (28) 78 (22)
4+ 34 (15) 6 (9) 32 (15) 0.30 0.41 25 (16) 47 (13) <0.05 <0.05
Parent marital status
Single 31 (13) 9 (14) 43 (19) 20 (12) 63 (18)
Cohabiting 25 (11) 11 (17) 22 (10) 13 (8) 45 (13)
Married 171 (73) 40 (61) 141 (64) 124 (77) 228 (63)
Other 7 (3) 6 (9) 16 (7) 0.05 0.34 5 (3) 24 (7) <0.05 0.19
Parent occupation (NS-SEC)
1 20 (9) 8 (13) 28 (14) 18 (12) 38 (12)
2 19 (9) 7 (12) 17 (9) 12 (8) 31 (10)
3 12 (6) 4 (7) 11 (6) 8 (5) 19 (6)
4 6 (3) 3 (5) 8 (4) 7 (5) 10 (3)
5 20 (9) 4 (7) 24 (12) 16 (11) 32 (10)
6 31 (14) 8 (13) 35 (18) 17 (11) 57 (17)
7 34 (16) 6 (10) 27 (14) 19 (13) 48 (15)
8 74 (34) 20 (33) 50 (25) 0.08 0.17 52 (35) 92 (28) 0.45 0.57
a: Adjusted for child age and IMD2007 score. †: Adjusted for child age only. Values which do not differ are marked with same superscript letter.
133 4.2.3.4 Predictive validity (multivariate predictors of MMR receipt)
Predictive validity analyses for individual items and for the attitudes scales are presented in Table 4.5 and Table 4.6. Predictive validity was demonstrated for a number of items and scales. In the complete regression model incorporating all the individual attitude items, the only predictors of both MMR dose 1 receipt and MMR dose 2 receipt were being in a black or minority ethnicity (BME) group (OR=2.21-3.92, p<0.001–p<0.05), having less experience of bad measles cases (OR=0.68-0.72, p<0.05), and anticipating less regret as a result of an MMR reaction (OR=1.53-1.76, p<0.01–p<0.05).
Believing that measles is serious (OR=1.49, p<0.05), having more experience of bad MMR reactions (OR=0.66, p<0.05), and having less preference for single vaccines (OR=1.58, p<0.01) predicted receipt of MMR dose 1 only. Younger child age (OR=0.89, p<0.01) predicted receipt of MMR dose 2 only.
In the complete regression model incorporating the attitude scales, being in a BME group (OR=1.94-4.15, p<0.001–p<0.05), having positive MMR attitudes (OR=1.63-1.97, p<0.05), and having positive social attitudes (OR=1.64-1.72, p<0.01) all predicted both MMR dose 1 receipt and MMR dose 2 receipt. Positive information source beliefs predicted dose 1 receipt only (OR=1.76, p<0.05), and younger child age predicted dose 2 receipt only (OR=0.91, p<0.01).
134 Table 4.5: Predictive validity (linear regressions of individual items onto MMR status - on time versus none)
Predictor
Odds ratios
MMR dose 1 MMR dose 2
Step 1 Step 2 Step 3 Step 1 Step 2 Step 3
n 322 359
Nagelkerke R2 0.002 0.11 0.38 0.01 0.21 0.39
Child age 1.02 1.02 0.97 0.97 0.94 0.89*
IMD2007 score 1.00 1.01 1.01 0.99 1.00 0.99
Parent age 1.09 1.33‡ 0.93 1.06
Parent BME ethnicity 2.65*** 2.21* 4.42*** 3.92***
Parent marital status†
Cohabiting 1.71 1.75 0.81 0.65
Married 1.55 1.57 1.53 1.54
Other 0.51 0.49 0.63 0.62
Parent education††
GCSE/O-level 1.27 1.13 0.70 0.82
A-level 1.30 1.40 1.59 2.38
Diploma 1.40 1.24 0.80 0.92
Degree 1.58 1.67 1.09 1.50
Postgraduate 2.53 2.20 1.93 2.88
Other 1.23 0.90 0.87 0.71
Number of children 1.10 1.30 1.13 1.29
Parent occupation (NS-SEC coded) 1.05 1.01 1.02 0.98
MMR has serious side effects 1.17 1.16
Measles is a severe illness 1.49* 1.20
MMR protects against measles 0.92 1.06
Seen/heard of bad MMR reactions 0.66* 0.84
Seen/heard of bad measles cases 0.68* 0.72*
Fear immune overload 0.90 1.15
Prefer separate vaccines 1.58** 1.26
135 Predictor
Odds ratios
MMR dose 1 MMR dose 2
Step 1 Step 2 Step 3 Step 1 Step 2 Step 3
Without MMR, measles is likely 1.04 1.09
Prefer natural immunity 0.79 0.84
Trust GP/nurse’s MMR advice 0.86 0.83
Scientific MMR research is adequate 1.05 0.97
Like social benefit of immunising 1.45‡ 1.55‡
Done personal research 1.08 1.18
Rejected MMR for other children 1.28‡ 1.32
Does not believe media MMR stories 1.23 0.99
Anticipate regretting measles 1.12 1.34
Anticipate regretting MMR reaction 1.76** 1.53*
Thought about MMR pre-birth 1.21 1.04
Perceive practical barriers to MMR 1.12 0.84
Perceive pro-MMR social norms 1.24 1.17
†: Reference category = Single. ††: Reference category = No qualification. ***: p<0.001. **: p<0.01. *: p<0.05. ‡: p<0.10
136 Table 4.6: Predictive validity (linear regression of scales onto MMR status - on time versus none)
Predictor
Odds ratios
MMR dose 1 MMR dose 2
Step 1 Step 2 Step 3 Step 1 Step 2 Step 3
n 373 415
Nagelkerke R2 0.002 0.09 0.28 0.01 0.20 0.31
Child age 1.01 1.01 0.97 0.96 0.94 0.91**
IMD2007 score 1.00 1.01 1.01 1.00 1.00 0.99
Parent age 1.09 1.24‡ 0.95 1.01
Parent BME ethnicity 2.26** 1.94* 4.05*** 4.15***
Parent marital status†
Cohabiting 2.15‡ 2.13 0.88 0.78
Married 1.93* 1.78 1.58 1.53
Other 0.76 0.64 0.79 0.67
Parent education††
GCSE/O-level 1.10 1.03 0.69 0.75
A-level 1.46 1.61 1.92 2.27
Diploma 1.17 1.46 0.77 0.92
Degree 1.30 1.46 1.02 1.11
Postgraduate 1.78 1.80 1.49 1.68
Other 1.76 1.51 1.41 1.29
Number of children 1.11 1.19 1.14 1.18
Parent occupation (NS-SEC coded) 1.05 1.04 0.99 0.96
MMR beliefs 1.63* 1.97*
Measles beliefs 0.82 0.87
Social and parenting beliefs 1.72** 1.64**
Information source beliefs 1.76* 1.16
Practicalities 1.26 1.09
†: Reference category = Single. ††: Reference category = No qualification. ***: p<0.001. **: p<0.01. *: p<0.05. ‡: p<0.10
137 4.2.4 Discussion
4.2.4.1 Instrument quality, main findings and comparison with findings from other studies