## 4.6Optional questions

These questions are optional; e.g., if you need more practice, or you are studying for the exam. (Answers are available in Sect. A.4.)

### 4.6.1Optional: Odds ratios

This question has a video solution in the online book, so you can hear and see the solution.

The impact of environmental toxins is not well understood. A 2012 Australian observational study examined the association between the environmental exposure of toxins and sporadic motor neuron disease (SMND).

A total of $$380$$ SMND cases were studied, and $$377$$ controls. Of the $$380$$ SMND cases, $$60$$ had worked with metal in the past; of the $$377$$ controls, $$33$$ had worked with metal in the past.

1. What is the direction of this study?
2. Construct a two-way table showing the relationship between disease group, and whether not the specific person had worked with metal, in the sample.
3. Using your table, compute the odds that a person with SMND had worked with metal. Interpret what this means.
4. Using your table, compute the odds that a person without SMND had worked with metal. Interpret what this means.
5. How many times greater is the odds that a person with SMND having worked with metal, compared to the odds that a person without SMND having worked with metal? (This is an odds ratio.)
6. Using your table, compute the percentage of people with SMND that had worked with metal.
7. Using your table, compute the percentage of people without SMND that had worked with metal.
8. A newspaper report states SMND rates are almost the same between those worked with metal and those who did not. Do you agree or disagree?
9. Sketch a bar chart to display the data.

### 4.6.2Optional: Percentages and odds

Endotracheal intubation (ETI) is a method for maintaining an open airway or as a method for administering certain drugs for ill patients.

ETI is widely used for airway management of children in the out-of-hospital setting, but for many years little evidence was available on the effect of using ETI. Table 4.5 comes from a study of using different types of endotracheal intubation on children. Use the table to answer the questions that follow.

TABLE 4.5: Airway management data
BVM group
ETI group
Number % Number %
Normal, no change from baseline 39 (10%) 33 (8%)
No change from baseline 33 (8%) 25 (6%)
Mild disability 20 (5%) 27 (6%)
Moderate disability 6 (1%) 7 (2%)
Severe disability 10 (2%) 6 (1%)
Coma/vegetative 15 (4%) 12 (3%)
Death 281 (70%) 306 (74%)
Total 404 416
1. Among people treated with BVM ('ball valve mask'), compute the percentage that died (to the nearest percent).

2. Among the people treated with BVM, what are the odds that someone died? Explain which one of the following calculations is correct, and explain what the other calculations compute, and hence why they are wrong.

1. Among people treated with ETI, the odds that someone died is $$2.78$$. What does this mean?
2. How many times greater is the odds that a person treated with ETI died compared to the odds that a person treated with BVM died?

### 4.6.4Optional: Percentages

Industries often produce unpleasant odours, but the effects on the health of those impacted is not always clear. Furthermore, different communities are impacted in different ways. One study researched the RQ:

In Carson, California, does the odour from nearby petroleum refineries impact residents in different areas differently?

One aim of the study was to determine if the reports of unpleasant odours were related to measurements made by a dynamic olfactometer (an 'electronic nose'). Part of the data collected are shown in Table 4.6.

TABLE 4.6: Data on the impact of a local industry
Area I Area II Area III Total
At least once a week 48 48 16 112
Between once a week and once a month 23 21 10 54
Less often than once a month 14 14 20 48
Not at all 12 12 53 77
1. Explain whether the study is observational or experimental (true or quasi).
2. What would be the meaning of column percentages for this table?
3. What would be the meaning of row percentages for this table?
4. Would row or column percentages probably be more sensible to use? Why?
5. Compute the percentage of residents in the sample overall not detecting the odour at all.

### References

Deane M, Sanders G. Annoyance and health reactions to odor from refineries and other industries in Carson, California 1972. Environmental Research. 1978;15:119–32.
Gausche M, Lewis RJ, Stratton SJ, Haynes BE, Gunter CS, Goodrich SM, et al. Effect of out-of-hospital pediatric endotracheal intubation on survival and neurological outcome: A controlled clinical trial. Journal of the American Medical Association. 2000;283(6):783–90.
Pamphlett R. Exposure to environmental toxins and the risk of sporadic motor neuron disease: An expanded Australian case–control study. European Journal of Neurology. 2012;19:1343–8.