## Tuesday, July 28, 2020

### Charting Dual Average Percentages as Linear Trends

An excellent example of presenting two sets of data from multiple sources over time can be found in the presentation of polling data on FiveThirtyEight.*

The point of  this post is to identify a useful way to present data from multiple sources over time.

The charts are continually updated as data from new polls are received*. The data for each of the two main candidates are plotted and a trend line shows the averages for each candidate. From left to right we see the progress based on the dates of each poll.

Notes about the chart and the data

1. Percentages can be averaged and yield a meaningful and easy way to interpret multiple sets of data.
2. The narrowing and widening of the trend lines offers a quick glance at what is happening for each candidate or data source.
3. Averaging polls from multiple sources helps avoid bias due to emphasizing preferred outcomes.
4. Below the chart are tables of data showing important information useful to research methods
4.1 Dates are listed and are clearly important as events can change what happens.
4.2 The data source is listed, which adds to confidence that we are not dealing with hidden data.
4.3 The size of the sample is important to determine accuracy.
4.4 The type of sample is important. So, LV = likely voters is more important to voting outcomes than is a general population sample. Other types of samples are listed.
4.5 The results are listed along with the difference presented as a net result. On this day Biden was 8 points ahead of Trump.

It is worth noting in polls like this that events can change the trend. That is, just because Biden is consistently ahead of Trump does not mean this will be true later in the year. Also, it is important to remember that in the US, the popular vote does not determine who wins in a close election. As in 2016, the president won the electoral vote but lost the popular vote. Thus, polling based on electoral votes will be more useful for predicting election outcomes.

*Nate Silver is the founder of the website FiveThirtyEight. The story of Nate Silver's excellent predictions can be found in The Signal and the Noise: Why So Many Predictions Fail--but Some Don't

**Data Note that, in science writing, the word data takes a plural verb so, "data are" not "data is." To refer to a single unit of data, use the word datum. See dictionary.com In nonscience writing, data has been used as singular or plural.

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Cite this blog post

Sutton, G.W. (2020, July 28). Charting Dual Average Percentages as Linear Trends. Assessment, Statistics, & Research. https://statistics.suttong.com /2020/07/charting-dual-average-percentages-as.html

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