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Statistics in Nursing: Case Study – Odds Ratio and P-values

Statistics in Nursing: Case Study – Odds Ratio and P-values

Statistics in Nursing: Case Study – Odds Ratio and P-values

See the data report provided below. Calculate the Odds Ratios and p-values for each potential source of food-poisoning. See if you can figure out the likely source of the food-poisoning in this case. Comment on the response of at least one other person in the class. See if you as a group can come to a consensus on the most likely source of the food-poisoning outbreak. (See the Odds Ratio Scenario 2 above – the free online calculator is hyperlinked in this document.)

Campus Food-Poisoning Incident

In scenario two we used the Odds Ratio and the online calculator to rule out the new Salad Bar as the likely source of an outbreak of food poisoning on campus on May 4th. In that scenario we found the following data for those who ate lunch and dinner on campus that day…

1. Salad Bar
ill exposed/total ill = 9/30
well exposed/total well = 36/100

 

Finding: We found that those eating at the salad bar were less likely to be ill than those who did not. So we have decided to re-open our investigation. We have the following additional data of those who ate on campus and were ill or well….:

2. Soup
ill exposed/total ill = 12/30

Well exposed/total well = 28/100

  1. Pizza

ill exposed/total ill = 7/30

well exposed/total well = 35/100

 

  1. Pasta

ill exposed/total ill = 2/30

well exposed/total well = 40/100

 

  1. Sandwich

ill exposed/total ill = 18/30

well exposed/total well = 18/100

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Food Poisoning and the Odds Ratio
Dr. Michael Landry

Consider the following scenario. We have had an outbreak of food poisoning on campus. We in this class speculate, well it seems that those who are in the infirmary are all talking about the new salad bar that just opened. So we decide to investigate. Here is our data and how we might use the odds ratio in this situation.

Findings: Students who ate at the salad bar on May 4th

ill exposed/total ill = 9/30

well exposed/total well = 36/100

We want to find the measure of association between the illness and the salad bar – and the p value.

We use the following Odds Ratio free calculator (data entered in screenshot below)

https://www.medcalc.org/calc/odds_ratio.php

Here is the Process:

  1. At the top we have the “bad” outcomes.
    Start with the number in the exposed group who are ill. This is 9.

Enter the number who are ill from the control (non-exposed) group. This is 30-9 = 21.

  1. In the middle we have the “good” outcomes.
    Start with the number who are healthy in the exposed group. This is 36.

Enter the number who are healthy in the control (non-exposed) group: This is 100-36 = 64.

  1. As we consider the numbers presented we can see that 45 (9 + 36) were exposed. Of these 9 became ill.

The risk of becoming ill from the salad bar was therefore 9/45 or .20. We also see that 85 (30-9 + 100-36)  were not exposed and of these 21 (30-9) became ill. The risk of becoming ill even though one did not eat at the salad bar is therefore 21/85 or .247.

  1. The Odds ratio is calculated at .76 in the free calculator (as noted below). This indicates that the odds of becoming ill from eating at the salad bar is about .76 times as great (it is lower) as the odds of not eating at the salad bar. So we can think of this as perhaps ruling out that something at the salad bar is the problem.

And we have p = .55 indicating there is essentially no significant difference in risk for these groups. My

basic finding would be that the salad bar therefore is not the source of the illness. We need to look for

another source… (you may need to enlargen the screenshot to see these details…