4.6 Optional 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.)

4.6.1 Optional: 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 (Pamphlett 2012) 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.

What is the direction of this study?
Construct a two-way table showing the relationship between disease group, and whether not the specific person had worked with metal, in the sample.
Using your table, compute the odds that a person with SMND had worked with metal. Interpret what this means.
Using your table, compute the odds that a person without SMND had worked with metal. Interpret what this means.
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.)
Using your table, compute the percentage of people with SMND that had worked with metal.
Using your table, compute the percentage of people without SMND that had worked with metal.
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?
Sketch a bar chart to display the data.

4.6.2 Optional: 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 (Gausche et al. 2000) 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

Among people treated with BVM ('ball valve mask'), compute the percentage that died (to the nearest percent).
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.

281/(404-281) = 2.28 (404-281)/404 = 0.304 281/404 = 0.696 (281+306) / ( 404+416) = 0.716 281/306 = 0.918

Among people treated with ETI, the odds that someone died is \(2.78\). What does this mean?

A patient treated with ETI has a 2.78% chance of dying. 2.78% of patients treated with ETI in the study died. For patients treated with ETI, about 278 die for every 100 that survive. For patients treated with ETI, about 278 live for every 100 that die.
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?

4.6.3 Optional: Odds

4.6.4 Optional: 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 (Deane and Sanders 1978) 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

Explain whether the study is observational or experimental (true or quasi).
What would be the meaning of column percentages for this table?
What would be the meaning of row percentages for this table?
Would row or column percentages probably be more sensible to use? Why?
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.