 unpaired ttest (also known as the student's ttest) and the paired ttest
both assume that analysed data is from a normal distribution
 unpaired
ttest
 applied to two independent groups e.g. diabetic patients versus
nondiabetics
 sample size from the two groups may or may not be equal
 in
addition to the assumption that the data is from a normal distribution, there
is also the assumption that the standard deviation (SD)s is approximately the
same in both groups
 ttest compares the means of the two groups of data
 the test determines whether the data has come from the same population or not
 the
mean difference is calculated (this can be a positive or negative value); also
a 95% confidence interval for the mean difference is calculated. A pvalue is
calculated where p is the probability of a falsepositive event. An example relating
to data regarding patients with diabetes in chronic heart failure
variable  diabetes
Mean (SD)  No diabetes Mean (SD)  Mean
difference 95% CI  pvalue  Age (years)  68.8
(8.7)  73.3 (9.0)  4.2 (3.1, 5.3)  p <0.0001  Heart
rate (bpm)  75 (15.0)  76.1 (14.9)  1.1 (0.9, 3.1)  p
0.85 
 in the example
 age of patient  the mean
difference in age is 4.2 years with a 95% confidence interval of 3.1 years to
5.3years. The pvalue for this relationship is <0.0001 which indicates that
falsepositive rate is very low (i.e. this difference is unlikely to be due to
chance). The data therefore suggests that patients with diabetes and chronic heart
failure tend to be younger than patients without diabetes and chronic heart failure
 heart
rate  in this example the mean difference is 1.1 years with a 95% confidence
interval of 0.9 years to 3.1 years. The pvalue for this relationship is 0.85
that indicates that the two groups are not statistically different from each other,
i.e. the observed difference is likely to be due to chance
 a nonsignificant
pvalue may occur because either
 there are no differences between the two
study groups
 the study did not have sufficent 'power' to show a difference
between the two study groups  with sufficiently large sample sizes then any difference
can be shown to be statistically different
 paired
ttest
 data is derived from study subjects who have been measured at two
time points (so each individual has two measurements). The two measurements generally
are before and after a treatment intervention
 95% confidence interval
is derived from the difference between the two sets of paired observations
Reference:  Doctor
(March 22nd 2005):3335.
