We have already seen that we found the ovens in use in almost half of the 10,000 visits. But we cannot conclude that in any given moment in Central America half of the solar ovens of CASEP are in use. Why?
One reason is that we almost always made our visits (80% of the time), when there was sufficient sun to cook. The percentage of days in Central America during which you are able to cook using the sun is less than 80%. Therefore, the average use would be less.
Since we do not know the average number of sunny days during the last 10 years in the 60 communities where we worked, we are restricted to the question: What is the use when there is good sun? According to our database, the use = 54%. But again we cannot conclude that on any given sunny day in Central America, 54% of the solar ovens are in use Why?
In any analysis, there are errors or systematic effects that are sometimes difficult to deduce, measure, or control. A phenomenon that affects us - that makes it difficult for us to arrive at an absolute measurement of use - is that there is a tendency among the women to use the oven the day that they know the promoter is going to make her visit. Since the principal goal of the visits is to encourage - not to measure the use - it is a bit difficult to control this systematic "error."
Of the 68% of the Owners that use it, are there at least 30% who use it on sunny days?
To say that use = 54% does not take into account the distribution among the owners. Does everyone use it 54% of the time? Or are there some who always use it and others, never?
Conclusion
The absolute measurements give us the idea that the use is high, but it is difficult to quantify the systematic errors. We reduce the problem of systematic errors if we look for relative measurements. What does use depend on?