Interaction with Water and Sanitation
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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.sav
2. Click on Statistics, Compare Means, and Means.
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 labeled Next to move to Layer 2 of 2.
5. In layer 2 of the Independent variable box, enter latcat (percent using safe latrines), then click OK.
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.sav
2. Click on Graphs, Line, Multiple, and Define.
3. In the box labeled Lines Represent, click on the dot called Other summary function and from the list of variables on the left hand side, enter acprvfm into the box using the arrow key.
4. For the box labeled Category axis, enter the variable wathcat using the arrow key.
5. For the box labeled Define lines by:, enter the variable latcat using the arrow key.
6. Add a title using the Title button, 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.
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