# Statistical Significance of Observed Reduction

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• #249945

briand
Participant

I was hoping someone might be able to help. What statistical analysis should I do to show if an observed reduction is significant of not?

For example if there were 315 presentations over a given time period, then after X event over a similar time period there were 260 presentations. How would I go about working out if this 17.5% decrease was statistically significant?

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#249953

Robert Butler
Participant

The question you are asking is one of a comparison of two proportions.  Specifically you want to know if the difference between 100% and 82.5% is statistically significant for a sample of 315.

The key is to recognize that the proportions are 315/315 and 260/315.  In other words, with a baseline of 315 in both cases we have a situation where there are 315 presentations and 315 successes and in the second case we have 260 presentations which means there are 260 successes and 55 “failures”.

As a result, you have a 2×2 table with 0 failures and 315 successes in the first case and 55 failures and 260 successes in the second case.  If you set this up and run the analysis you will find you have a p-value of < .0001 – so, there is a statistically significant difference between 100% and 82.5% for a base of 315 samples.

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#249969

briand
Participant

Thank you, thats a great help

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