Significance of Handwashing Frequency
Date: 02/23/2002 at 01:11:30 From: Adam Subject: Determining Significance of Handwashing Frequency I'm doing an experiment for Psychology class. I went to a public restroom and took data on how many people washed their hands both when there was an observer present and when there wasn't. 25 people were observed with a visible observer present, and 25 without. Out of the people accompanied by an observer, 23 washed their hands and 2 did not. When an observer was not present, 12 washed their hands and 13 did not. I need to determine whether the absence of an observer (i.e. if the bathroom-user is alone) makes a statistically significant difference. I think I need to use a chi-squared test for this, but I don't know how to do one. Could you please show me the procedure for doing one? Thanks a lot! - Adam
Date: 02/23/2002 at 03:33:58 From: Doctor Schwa Subject: Re: Determining Significance of Handwashing Frequency Hi Adam, your null hypothesis in this case is that people are equally likely to wash their hands whether or not an observer is present. Based on your data, the overall probability of handwashing is 35/50 or 70%. Thus, in each case, you'd predict 70% of 25 would wash their hands: 17.5 people. You can make a table: ACTUAL wash not wash observed 23 2 not obs. 12 13 PREDICTED wash not wash observed 17.5 7.5 not obs. 17.5 7.5 Now, in each of the 4 spaces, calculate (actual - predicted)^2, and divide the result by the predicted number for that space. Then add up those 4 numbers, and you'll have the chi-squared statistic. In this case, there's one degree of freedom (once we figured out that one 70% number, we could fill in the whole PREDICTED table). So, to find out if the result is significant, compare it to a table of values for chi-squared with one degree of freedom. I hope that helps clear things up. If not, you should try searching our Dr. Math archives for the keyword chi-square . http://mathforum.org/mathgrepform.html You'll find a few more, different explanations of how to do this. If that doesn't help, please write us back. Enjoy, - Doctor Schwa, The Math Forum http://mathforum.org/dr.math/
Date: 02/24/2002 at 00:38:22 From: Adam Subject: Determining Significance of Handwashing Frequency Thank you very much for your help. My results turned out to be significant at the 95% (p=.05) level. As a bonus, I can now boast to my math and psychology teachers that I know something they don't. Thanks a lot, Dr.!
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