| Course | HS340 Epidemiology |
|---|---|
| Unit | Unit 4 |
| Paper type | Two by two table |
| Length | About 570 words, 5 pages double-spaced |
| Format | APA 7 student paper |
| School | Purdue University Global |
| Program | BS in Health Science |
| Updated | October 2026 |
Free sample paper for HS340 Unit 4
Small Turtles, Four Cells: A Unit 4 Two by Two Table for a Composite Salmonella Case-Control Study
[Student Name]
Purdue University Global
HS340: Epidemiology
Unit 4 Two by Two Table
[Instructor Name]
[Date]
The cases, controls and counts are composite; the review of turtle-associated illness and the methods sources cited are real.
The Table
Rows are exposure; columns are case status.
Exposed to a pet turtle, cases: a = [19]. Exposed to a pet turtle, controls: b = [14]. Not exposed, cases: c = [29]. Not exposed, controls: d = [130]. Column totals: [48] cases and [144] controls.
Row totals are [33] exposed and [159] unexposed, but they carry no meaning here. The investigators chose three controls for every case, so the share of cases in any row is an artifact of that choice. If they had chosen four controls per case, every row proportion would change while the odds ratio stayed about the same (Schulz & Grimes, 2002).
In a case-control table, only the column totals are fixed by nature of the study, so risks cannot be read across the rows; only odds can be compared.
The Calculation
Odds of exposure among cases: 19 divided by 29, or 0.655. Odds of exposure among controls: 14 divided by 130, or 0.108. Odds ratio: 0.655 divided by 0.108, or equivalently the cross-product, 19 times 130 over 29 times 14, which is 2,470 over 406, or 6.08.
The Confidence Interval
The interval is built on logarithms (Centers for Disease Control and Prevention, 2012). Taking the natural log of 6.08 gives 1.806. Its standard error comes from adding the reciprocals of the four cells and taking the square root: 1/19 plus 1/29 plus 1/14 plus 1/130, which is 0.166, and its square root is 0.408. Multiplying by 1.96 gives 0.799. The log interval runs from 1.007 to 2.605, and converting back gives 2.7 to 13.5.
Reading the Result
The odds of turtle contact were about six times higher among cases than among controls. Because Salmonella infection is uncommon in the population, the odds ratio is a close stand-in for the relative risk here, meaning people who touched a pet turtle were roughly six times as likely to fall ill as those who did not. The interval is wide, reflecting only [33] exposed people, but its lower bound of 2.7 sits well clear of 1, which makes chance a poor explanation.
The Share of Cases the Exposure Explains
Among cases, 19 of 48, or 39.6 percent, were exposed. The fraction of cases attributable to turtle contact is that proportion times (OR minus 1) over OR: 0.396 times 5.08 over 6.08, or about 33 percent. If the association is causal, roughly a third of cases in this cluster would not have occurred without turtle contact.
What the Table Cannot Settle
The table does not show that turtles caused the illnesses. Cases might recall a pet more readily than controls, since they had been asked about animals by a clinician already. Families who keep turtles may also differ, for example in having young children who put hands in mouths. Matching the strain from cases to samples from turtle tank water would strengthen the link. Small turtles have repeatedly been tied to Salmonella outbreaks in children, despite a long-standing federal restriction on selling turtles with shells under four inches (Bosch et al., 2016), which makes the finding plausible but not proven by this table alone.
References
Bosch, S., Tauxe, R. V., & Behravesh, C. B. (2016). Turtle-associated salmonellosis, United States, 2006-2014. Emerging Infectious Diseases, 22(7), 1149-1155. https://doi.org/10.3201/eid2207.150685
Centers for Disease Control and Prevention. (2012). Principles of epidemiology in public health practice: An introduction to applied epidemiology and biostatistics (3rd ed.). U.S. Department of Health and Human Services.
Schulz, K. F., & Grimes, D. A. (2002). Case-control studies: Research in reverse. The Lancet, 359(9304), 431-434. https://doi.org/10.1016/S0140-6736(02)07605-5
Reading the HS 340 Unit 4 instructions
Unit 4 typically gives the counts from a case-control or cohort study and asks you to construct a two by two table, calculate the appropriate measure of association with a confidence interval and interpret the result. Worksheets usually fit on one to two pages, with the calculation shown. Instructors look for exposure in the rows and outcome in the columns, cells labeled, the correct measure for the design and an interpretation in plain words. Many sections ask why a relative risk cannot be calculated from a case-control table and expect a sentence on what the interval means. Some add an attributable fraction. Cite a methods source for the formulas and, where useful, published evidence on the exposure.
How this HS340 Unit 4 example is built
The worksheet presents the table as four labeled cells with column totals and explains why the row totals carry no meaning. The highlighted sentence states that only odds can be compared in a case-control table. The calculation section gives the odds of exposure in each group and the cross-product. The confidence interval is worked step by step on the log scale, from the natural log of the odds ratio through the standard error to the converted limits. A reading section interprets the odds ratio as an approximate relative risk because the infection is rare and comments on the interval's width. The attributable fraction among cases follows. A closing section names recall and other family differences as limits and cites a real review of turtle-associated illness.
Where the points sit in the HS340 Unit 4 rubric
Two by two worksheets are usually marked on correct table construction, correct measure for the design, accurate calculation, interpretation and explanation of limits. Choosing the right measure for the design often carries the most weight, because a risk ratio computed from a case-control table is wrong however neat the arithmetic. Table construction earns points for labeled cells and margins. Calculation credit covers the odds ratio and an interval computed correctly. Interpretation is rewarded when the result is stated in words a nonspecialist could follow, including what the interval says. Limits earn credit when they name specific biases in the study. Showing every step lets a grader award partial credit even after an arithmetic slip.
Common HS340 Unit 4 mistakes, and how to avoid them
The most frequent error is computing a relative risk across the rows of a case-control table. Another is placing outcome in the rows and exposure in the columns without adjusting the formula, which turns the odds ratio upside down. Some students report the odds ratio without an interval or describe the interval as the range of true values. Others interpret the odds ratio as proof of causation. Rounding intermediate logarithms too early can shift the interval noticeably. If your table comes from a different study, perhaps a cohort with attack rates or a matched design, pass the counts along and we will work through choosing the measure, compute the interval and word the interpretation.
Get an HS340 Unit 4 example written to your instructions
Forward the counts or line list behind the HS340 Unit 4 table, with the questions exactly as posted and the rubric. The desk letters the cells, chooses the measure the sampling permits and reads it in plain words. That first custom sample is free and reaches the inbox in 24-48 hours, the interval computed and its width explained. The paper above is an original model document written by our desk, not a submitted student paper and not an official Purdue University Global document.
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HS340 Unit 4 questions, answered
Why not calculate a relative risk from the Salmonella table?
Because the investigators decided how many controls to enroll. With three controls per case, the share of exposed people who are cases reflects that decision rather than how often exposed people fell ill. The odds ratio does not depend on the sampling fraction, which is why case-control tables report it. For an uncommon infection, it also approximates the relative risk a cohort would have found.
Does the order of rows and columns matter?
Only if the formulas do not follow it. The standard cross-product assumes exposed people in the top row and cases in the left column. Swapping one of those, so the unexposed row sits on top, turns 6.1 into 0.16 without any warning. Labeling each margin in words and lettering the cells makes the slip much harder to commit, and a grader can check the layout at a glance.
How should a confidence interval this wide be described?
As an honest range, with its cause named. An interval from 2.7 to 13.5 says the data are consistent with a modest association and with a very strong one, and the width comes mostly from small cell counts. It still excludes 1.0, so chance alone is an unlikely explanation. A larger study, or pooled data from several states, would narrow it considerably.
Where can I find a free HS340 Unit 4 sample paper?
The full Unit 4 two by two table is here, with an odds ratio and interval for pet turtle contact in a composite Salmonella study.
What is an attributable fraction among cases?
The share of cases that would not have occurred without the exposure, if the association is causal. It combines how many cases were exposed with the strength of the association.