Mother and Child Health: Research Methods

G.J.Ebrahim

Editor

Journal of Tropical Pediatrics, Oxford University Press.

Programmes Menu of Epi-Info

The Normal Distribution

• A histogram provides a view of how the values of a variable are distributed.

Number of values and the Gausian shape

• All these histograms illustrate a Normal distribution. As the number increases the Gausian shape becomes more obvious

Scatterplot to show relationship between two numeric variables

• A scatterplot of dose of anaesthetic and time to recover from anaesthesia, with a regression line and 95% confidence intervals as well as outliers.

Outliers

• An outlier is an unusually high value of Y for a given value of X. Outliers exert an influence on the regression line.

Rates, Proportions and Ratios

Is numerator included in the denominator?

Yes No

Is time included in denominator?

Yes No

MEASURE: Rate Proportion Ratio

EXAMPLE: Incidence Prevalence Maternal Mortality

Rate Rate ratio

Absolute risk, Probability of survival and Odds ratio

Absolute Risk Survival Probability

Odds

0.90 1−0.90 = 0.10 0.90÷(1−0.9) =9.0

0.75 1−0.75 = 0.25 0.75÷(1−0.75) = 3.0

0.50 1−0.50=0.50 0.50÷(1−0.50)=1.0

0.25 1−0.25=0.75 0.25÷(1−0.25)=0.33

0.10 1−0.10=0.90 0.10÷(1−0.10)=0.11

0.01 1−0.01=0.99 0.01÷(1−0.01)=0.01

Calculating Numbers Needed to Treat (NNT)

• Calculate the proportion of people who have the outcome in the intervention group. (Exp.Grp.Evnt Rate; EGER )

• Calculate the proportion of people who have the outcome in the control (placebo) group. (Cntrl. Grp.Event Rate; CGER)

• Calculate the difference between EGER and CGER to obtain Absolute Risk Reduction (ARR)

• Then NNT = 1÷ARR