648 Obstetrics and Gynecology Board Review •••
❍ What is meant by the phrase “measures of central tendency”?
Measures of central tendency refer to summary numbers that describe the middle of a distribution such as the mean,
median, or mode. The mean is the sum of the X values divided by the total number in the sample (n). The median is
the middle observation. The mode is the value of a numerical variable that occurs the most frequently.
❍ Define confidence interval.
The interval calculated from sample data that has a given probability (eg, 95%) that the unknown parameter (eg, a
mean or proportion) is contained within the interval. The confidence interval is the estimated range within which
it is likely (eg, with 95% probability) that the true point estimate (eg, the true population mean) exists. Confidence
intervals are usually expressed as 90%, 95%, or 99%. The width of the confidence interval is inversely related to
the sample size, indicating that as the amount of information about the population increases, the precision in the
estimate increases.
❍ What is meant by the effect size?
It is the magnitude of a difference or relationship. It is used in sample size calculations and for combining results
across studies as in a meta-analysis.
❍ What is the independent sample t-test?
The independent sample t-test (also known as the student’s t-test) is used to test the null hypothesis that two
independent (or unrelated) samples have the same mean. The t-test is used for quantitative data.
❍ What does ANOVA stand for?
ANOVA stands for analysis of variance; it is similar to but more robust than the t-test. It is a statistical procedure
that compares the means of two or more groups of subjects on one or more variables to determine if they are
(statistically) significantly different.
❍ What is a paired or dependent-sample t-test?
The paired (or dependent sample) t-test is used to compare the means of two paired observations on the same
individuals. Quantitative data collected in a before and after study design are analyzed using the paired t-test.
❍ What is the chi-square test (c^2 )?
It is a statistical test for contingency tables used to determine whether factors or characteristics are independent or
not associated with each other. It is also used to test the null hypothesis that proportions are equal.
❍ Describe the Fisher’s exact test.
It is a statistical test for contingency tables that are used when the sample size is too small to use the chi-square test.
A contingency table is used to display counts, or frequencies for two or more qualitative variables.
❍ What is the Mantel-Haenszel test?
The Mantel-Haenszel test is a statistical test of two or more 2 × 2 tables. It is used to compare survival
distributions or to control for confounding factors.
❍ What is RR?
RR is the ratio of the incidence of a given disease in an exposed or at-risk population to the incidence of the disease
in unexposed persons. It is calculated in cohort or prospective studies.