As we'll see later on, the higher the Confidence Level, the wider the Margin of Error will be. Also, a 95% Confidence Level means that 5% of the time the ACTUAL result will lay outside the Confidence Interval you've calculated. More on this soon...
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Confidence Level is a measure of how confident you are that the statistics you've calculated are accurate. There is no calculation involved in the Confidence Level. You choose your Confidence Level based on the degree of confidence required to satisfy your client. The most common Confidence Level for usability analysis, and the one we'll use in this example, is 95%. Others are:
Note our progress:
To be complete, statistical conclusions should always include:
But we need to have the Confidence Level in a different format - called the Significance Level - in order to continue our calculation. As you can see below, it's a pretty simple conversion. 95% Confidence Level equates to a .05 Significance Level.
| Calculated Data | Symbol | Formula | Value |
|---|---|---|---|
| Confidence Level | C | 95% | |
| Significance Level | α | 1-(C/100) | .05 |
At this point we have determined our Success Probability and our Confidence Level. The only thing left to do is calculate our Confidence Interval. This will take a few steps.