Interpretation of hypothesis generation results

Examine the results of the food frequency analysis using binomial probabilities below [original table available in the third tab of the Excel spreadsheet for Exercise 3 (Module 2 – Exercise 3)].

The table shows that multiple food items are above expected: lettuce (any and romaine), bell peppers (any and red), cherry tomatoes, mushrooms, hamburgers, bacon and strawberries.

Food Item Confirmed Cases Reference A* Reference B† Reference C++ Binomial Probability
   Yes  Prob  No  DK %Y +P CDC Food Atlas (2006/07) Waterloo Study (2008) Foodbook Canada (2015) Reference A Reference B Reference C
VEGETABLES
Any lettuce 6 0 0 1 100.00%   84.00% 82.40%   0.3513 0.3130
Romaine lettuce 6 0 0 1 100.00% 46.50%   48.80% 0.0101  

0.0135

Any bell peppers 5 0 1 1 83.33%   27.03% 63.60%   0.0063 0.2273
Red bell peppers 5 0 1 1 83.33% 29.50%     0.0095    
Orange bell peppers 4 0 1 1 80.00%            
Yellow bell peppers 1 0 1 1 50.00%            
Tomatoes (fresh, not grown at home) 5 1 1 0 85.71% 59.60% 72.51% 72.90% 0.1268 0.2797 0.2847
Cherry tomatoes 3 1 1 1 80.00%     32.80%     0.0389
Cucumbers 4 0 2 1 66.67% 46.90% 54.56% 62.90% 0.2046 0.2745 0.3232
Broccoli 4 0 2 1 66.67% 53.30% 43.38% 55.50% 0.2640 0.1703 0.2818
Mushrooms 5 0 1 1 83.33% 33.50% 36.46% 50.00% 0.0168 0.0246 0.0938
MEAT
Any beef (not including deli meat) 7 0 1 0 87.50%   78.65% 78.40%   0.3180 0.3146
Hamburgers 7 0 1 0 87.50%   35.88% 39.10%   0.0039 0.0068
Bacon 4 1 3 0 62.50% 46.30%   27.20% 0.1845   0.0322
Any chicken (not including deli meat) 6 1 1 0 87.50% 64.90% 91.43% 85.60% 0.1362 0.3662 0.3879
Whole or cut chicken pieces or parts 4 0 3 1 57.14%     70.00%     0.2269
DAIRY & EGGS
Ice cream / gelato 4 0 2 1 66.67% 59.20% 42.82% 42.00% 0.3067 0.1649 0.1570
Eggs 4 1 1 0 83.33% 75.40% 82.22% 80.70% 0.3597 0.4008 0.3963
Handling raw eggs 4 1 1 0 83.33%            
FRUIT
Strawberries 4 0 1 1 80.00% 45.00% 31.54% 49.60% 0.1128 0.0339 0.1525
Bananas 4 1 2 0 71.43% 70.30% 79.04% 76.70% 0.3181 0.2846 0.3026
OTHER
Cold breakfast cereal 3 1 2 0 66.67% 69.20%   54.30% 0.3263   0.2723

Other prepackaged snack food

(e.g., chips, pretzels, crackers, cookies, snack cakes)

4 1 1 0 83.33% 71.80%     0.3229    

*Based on Population Survey Atlas of Exposures, 2006-2007, United States Centers for Disease Control and Prevention

†Based on Waterloo Region, Ontario food consumption survey, November 2005 to March 2006

++Based on the Foodbook Survey, 2015, Public Health Agency of Canada

Question 2-5: What do these results mean? Why are multiple items identified? 

ANSWER

Items above expected levels “may” be a source and should be examined in further detail. Statistically, with so many questions on a hypothesis-generating questionnaire, there will be some that come up only by chance (especially with a small sample size like the one in this case study).

Some items may be above expected because they are “in season” (e.g., some summer fruits have artificially low “expected” values on food consumption surveys). In our case this may explain why strawberries are coming up; however, note that strawberries have been linked to E. coli O157:H7 outbreaks in the past.

On the other hand, it is important to keep in mind that some foods with high expected consumption levels (e.g., chicken) may not flag statistically, but could still be potential sources.

 

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