The Development of Circadian Rhythms in Human Infants

2.11 Statistical methods
For comparison between normally distributed sets of data Student's t tests have been
used, paired where appropriate. Student‟s t test was used, for example, in the
comparison of the mean age of mothers of participant infants with that of mothers
who declined, in this case unpaired (see results section 3.2.1). This method
determines whether two normally distributed sets of numbers could reasonably come
from the same population.
Data that were found to be positively skewed (such as reported melatonin and cortisol
lab values), were logged for analysis where required (see results chapters 5 & 6).
The non parametric tests Mann-Whitney U (for unpaired data) and the Wilcoxon
signed rank statistic were also used (for paired data). Mann Whitney U test shows
differences between overall distributions rather than between distribution means.
The Pearson's product moment r statistic was used to test for correlation between
variables, such as investigating the relationship between night and day melatonin.
Linear regression was used to investigate whether the change in one variable (x) was
associated with a change in a second variable (y) in order to predict y for a given value
of x. The equation y = α + βx was defined as the linear regression equation, where α is
the intercept and β is the regression coefficient. α is the value of the equation when x
= 0 and is the slope of the line. When x increases by one unit, y will change by β
units. This modelling was applied to temperature, sleep, melatonin, and cortisol data
in order to investigate the factors influencing these within the infants group (see
results chapter 9).
79