## So what exactly is a bad run?

As a rule of thumb, as the density altitude (DA) increases, so should your ET – the higher the altitude, the slower you go. A bad run is any run that has a slower ET at a lower altitude than an existing run.

Have a look at the following example runs entered into your weather station:

All of these runs ‘fit’ together – as the altitude increases, so does the ET. Now, let’s suppose you enter another run (number 4) into your weather station:

Now we have a problem. Run 4 does not make sense with the existing data as the ET is slower and the altitude higher than run 2. This is an example of a bad run.

## How do bad runs affect ET prediction?

As explained in a previous article, Weather Stations 101, a drag racing weather station predicts ET by graphing all your entered runs and using statistical analysis to calculate what your ET should be at a particular density altitude. If you’re interested, you can investigate this process in more detail by looking at regression analysis.

If we graph the runs from Table 1, we get the following:

From this example, we can see there is a nice and simple correlation between density altitude and ET, as represented by the straight line in the graph. Note – this is purely an example and is unlikely to be this simple.

If you had this data entered into your weather station, it would accurately predict your ET. For example, if the current altitude were 1200 ft, the predicted ET would be 10.01.