INTRODUCTION
2.5 Data Analysis
All subject data was transferred from paper data files to SPSS for Windows v 10. To facilitate data analysis five distinct files were created:-
a) baseline data file for 114 subjects
b) twelve months data file for 95 subjects
c) twenty-four month data file for 90 subjects
d) 24 subjects from the baseline data file who withdrew from the
study (see Appendix XII)
e) file for 79 subjects with data recorded at baseline, twelve and
twenty four months (see Appendix XIII). Descriptive statistics were used throughout the thesis.
Objective measures of asthma (spirometry and PEF), health status (AQLQ and Q score) psychological status (HAD score) prescribed asthma medication and asthma severity (BTS Guidelines treatment step) were collected throughout the study period (baseline, twelve and twenty- four months). Data were examined for relationships, (correlation design) using Spearman rank order correlation coefficient for non-parametric data. Non-parametric test were used for analysis with the AQLQ, Q score, HAD score and BTS treatment step as scales recorded ordinal level data. Data were examined for difference, (quasi-experimental design) from baseline to twelve months and baseline to twenty-four months using paired t test or Wilcoxon sign ranks for all subjects or using unpaired t tests or Mann-Whitney U tests for sub-groups.
2.5.1 Sub Division of Analysis
Data from the cohort was subdivided and explored at each stage by the following four sub-groups using experimental analysis.
Inner City versus Suburban Subjects
Subjects were divided by their place of residence and locality of GP practice. Two practices were situated in inner city areas with high Jarman (1983) deprivation scores (+18.7 and +13.45), two practices were situated in suburban areas having low scores for deprivation (-19.58 and –18.27). Differences between these two groups in relation to their spirometry and PEF, health status, psychological status and severity were explored by unpaired t test for parametric data or Mann-Whitney U tests for non-parametric data.
Severity - BTS Guidelines Treatment Step 1-2 versus 3-5
Severity of asthma was used to further sub divide subjects within the cohort as a whole. Subjects in low BTS treatment step (1 and 2) indicating mild to moderate asthma morbidity were assessed against subjects in the higher treatment steps (3-5) such subjects have less control over their morbidity and require increased medication in order to keep symptoms to a minimum (see Appendix III). Differences between these two groups in relation to their spirometry and PEF, health status, psychological status and place of residence were explored by unpaired t test for parametric data or Mann-Whitney U tests for non-parametric data.
Depression and Anxiety - Depressed Subjects versus Non- Depressed Subjects
Zigmond and Snaith, (1983) used three ranges of scores for depression and anxiety. Their scoring system comprised the following scores 0-7, for little to no disorder, 8-10 borderline and 11 plus significant depression or anxiety. In this study the cohort
was subdivided as 0-7 no depression or anxiety, 8 or above, possible depression or anxiety. Differences between these two groups in relation to their spirometry and PEF, health status, severity and place of residence were explored by unpaired t test for parametric data or Mann-Whitney U tests for non-parametric data.
Medication Changed - Inhaled Steroids Increased versus Inhaled Steroids Reduced/No change
Subjects’ prescription for asthma medication was reviewed at twelve and twenty-four months from baseline. The cohort was sub divided by subjects who had their inhaled steroid prescription increased at the time of the twelve month assessment (twenty-four month assessment) when compared to baseline prescription and subjects with inhaled steroid prescription remaining as at baseline or reduced from baseline. Differences between these two groups in relation to their spirometry and PEF, health status, psychological status, severity and place of residence were explored by unpaired t test for parametric data or Mann-Whitney U tests for non-parametric data.
2.5.2 Cross Sectional Data Baseline Data
Spearman rank order correlation coefficient was used to explore the relationship of subjective measures of asthma such as health status (as measured by Q score to AQLQ) and psychological status (as measured by HAD scale), to spirometry, PEF, severity (as measures by BTS guidelines treatment step) Binary logistic regression analysis was used to identify the variable most strongly linked to morbidity from spirometry, PEF, psychological status and severity.
Reproducibility of the Q Score
symptom domain of the AQLQ was examined by comparing baseline scores and scores collected from the random sub set of subjects at two weeks using Spearman rank order correlation co- efficient. Q score and AQLQ symptom scores correlate at baseline and at two weeks. The internal consistency of the Q score was measured at baseline and within the sub groups using Cronbach Alpha.
2.5.3 Longitudinal Data
These data were collated at twelve-month intervals throughout the study period (ie, on two subsequent occasions following initial assessment). Differences in spirometry, PEF, health status, psychological status, severity and medication uptake from baseline to twelve and baseline to twenty four months were explored for the whole cohort by paired t test for parametric data or Wilcoxon signed-ranks test for non-parametric data.
Longitudinal data was also explored by the sub-groups in 2.5.1. From baseline to twelve months and baseline to twenty-four months using unpaired t tests for parametric data and Mann- Whitney U tests for non-parametric data.
2.6 Ethical Approval
Approval was sort and gained from the local ethics committee, see Appendix XI for copy of letter of approval.