**Interaction
with Water and Sanitation**

An exercise using the Bangladesh data set (**bdeshc.sav**) will show how
water and sanitation can behave with the outcome variable of nutrition status (prevalence
of low arm circumference). Try the following steps in SPSS to see the mean outcome
of low arm circumference among children under 5 years by the independent variables water
source and latrine usage:

1. Open

bdeshc.sav2. Click on

Statistics, Compare Means, andMeans.3. In the Dependent variable box, enter the variable

acprvfm(prevalence of low arm circumference in male and female children) using the arrow key.4. In the Independent variable box, enter the variable

wathcat(percent using other water sources, >50% bad access to safe water, <50% good access to safe water) in layer 1, then click on the button labeledNextto move to Layer 2 of 2.5. In layer 2 of the Independent variable box, enter

latcat(percent using safe latrines), then clickOK.

INTERPRETATION: The results of the mean outcome scores show that there appears to be an large effect in the outcome if either latrine access or safe water access is improved, but little additional improvement if both are improved. This would be a very good example of interaction that might be referred to as synergism because improvement of only one factor is needed, instead of both to achieve the desired result. An important step to take now, is to graph the result to see the results visually. Try graphing the points from the results above, just use pencil and paper. It should look something like this:

It may also be useful to know how to graph these results using SPSS, for showing them in presentation. Here is an exercise to step you through the process of graphing in the SPSS program.

1. Open

bdeshc.sav2. Click on

Graphs, Line, Multiple,andDefine.3. In the box labeled

Lines Represent, click on the dot calledOther summary functionand from the list of variables on the left hand side, enteracprvfminto the box using the arrow key.4. For the box labeled

Category axis, enter the variablewathcatusing the arrow key.5. For the box labeled

Define lines by:, enter the variablelatcatusing the arrow key.6. Add a title using the

Titlebutton, if you like.7. Click on

OK.

INTERPRETATION: Again very similar to the hand graphed chart, the results show that there is a synergistic effect on nutrition status between improving water source and latrine use in Bangladesh. This means that there is an interaction, which is clearly represented by the difference in the slopes of the lines.