Opinion polls are often based on web data collection. This is not surprising, as it is a fast and cheap way of data collection. So, these polls can be carried out with a high frequency. The demonstration below shows the risk of drawing wrong conclusions.

There will be general elections in the country of Samplonia. The National Elderly Party (NEP) seems to do well in the campaign. An opinion poll is carried out to estimate the percentage of voters this party will attract. To determine how accurate the estimator is, sample selection is repeated a large number of times. The percentage of voters is computed for each sample. The distribution of all these estimates is shown in a histogram.

To carry out a simulation, you first set the sample size. You do that by clicking on the green square adjacent to Sample size. There are three possible sample sizes: 200, 400 or 800.

You can choose to select the sample from the whole population or just from those having Internet. You do that by clicking on the green square under Survey. You start the simulation by clicking on Start.

If you select samples from the complete population, you will see that the estimates are nicely concentrated arount the true value (25.4%).However, if you select samples from just the Internet-population, estimates will be systematically too low.

Why are the estimates too low? The reason is that the elderly are under-represented in the Internet-population. And the elderly typically vote NEP. So, then percentage of voters will be to low.