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A.V. García Luengo

Universidad de Almería (SPAIN)
Nowadays, the use of statistical packages in the resolution of problems is essential and we encourage the introduction in the classroom of materials, resources and new technologies in the education and learning of Statistics, for whose students of the University of Almeria (Spain) the following example was set out:
In many practical situations the population ratio of two characters for the most recent occasion may be of interest, for example, the ratio of corn acres to wheat acres, the ratio of liquid assets to total assets, or the population ratio of mean in mail surveys.
In sampling on two occasions, the population ratio from the current survey can be improved by using some information obtained in the previous survey. In these surveys some sample units may not response (or supply information) even after call backs.

Thus we examine the population ratio of mean in mail surveys for the current occasion in the context of sampling on two occasions when:
• There is non-response on both occasions.
• There is non-response only on first occasion.
• There is non-response only on second occasion.

We derive the saving in cost and an empirical study allows us to investigate the performance of the proposed strategy.

All the results have been carried out, using the statistical package "SPSS", the program Mathematica and language LaTeX. The Power-Point programme show and interpret the results to the students.