Statistics 2 Mod Exercise 4 - 5

Module Exercise 4: Association analysis

The exercise:

As a consultant health scientist with an engineering group you are contracted by the local authority of a small rural Australian town to investigate a recent outbreak among the residents of cramps and diarrhoea producing frequent loose and pale, malodorous, greasy stools.

You inspect the water supply and find it is pumped from a local creek directly into a large concrete reservoir without filtration, where it undergoes a 24-hour settlement before being subject to in-line chlorine gas disinfection and pumping to a gravity distribution tank on a hill, from which it enters the local reticulation to 1,900 residences. The chlorine residual in the reticulation has been checked by the district water chemist as 1 mg/l at the typical house standpipe, and the creek is able to supply the volume of water needed while other supplies tend to become unreliable in the dry season. Tests carried out four times a year show the water supplied to the houses to be free of E coli faecal indicator, and low in chemical pollutants such as heavy metals, BTEX and pesticides.

You take water samples at five of the houses for E coli (EC) and enterococci (Ent) indicator and while waiting for the results from a laboratory in a capital city initiate a health survey, receiving forms back from 630 of the 5,000 residents. Of the 400 residents who chose to drink town water, 220 became ill. However 87 of those who drank alternative water only (tank, borehole or bottled water) also reported becoming ill. Some residents comment that the town water sometimes has a “bad” taste and odour whereas others say they have always drunk the water and have never experienced any health problems.

Part 1:

Apply a non-parametric test of association to the survey data in order to decide whether there is a statistically significant association between drinking water from the piped water supply, and the incidence of diarrhoea. Here calculation of rates is not necessary as the appropriate statistical test automatically generates ratios (a basic type of rate).

Perform the test based on the method shown in the notes, using a calculator and any statistical tables needed, and present the results showing all tabulations and working. For those who wish to check their answer using Excel®, please note that this application gives the result as a statistical probability, p, and this will be a different value to that produced in your hand-worked answer.

Part 2: With regard to your survey result, describe one important short-term (immediate) recommendation and one important long-term recommendation you would make to Council.

Aim of exercise: To gain an insight as to the use of association analysis in the solution of a health problem with a single health state and one suspect risk factor, and to gain skills in using epidemiological results to develop important short- and long-term solutions to a public health problems.

Hints:

i. A good starting point in a public health investigation is to use the symptoms to identify what type of disease might be involved, then develop an initial hypothesis of how it might have spread.

ii. Next look at the environmental circumstances to see if known risk factors for the identified disease are present to support your hypothesis.

iii. finally carry out statistical analysis to support your hypothesis and to produce some figures to enable managers to motivate for resources in order to take action.

iv. Finally send of laboratory samples for pathogens to confirm your hypothesis.

Values of chi-squared to act as a standard for comparison.

Module Exercise 5: Relative risk analysis

Exercise:

146 cyclists attended a convention dinner at a hotel and on returning to their rooms a few hours afterwards 84 started to feel nauseous and began vomiting, without accompanying fever or diarrhoea. With a team of trained interviewers you visit all of the players and collect the following information:

Food eaten	Cyclists ill	Cyclists not ill
Bruschetta with liver pate	38	82
Corn chowder	18	44
Thai fish cakes	22	43
Chicken in aspic	79	31
Greek salad	70	70
Fruit flan	38	6
Coffee	29	31
Tea	33	30

Part1:

Showing full tabulations and working, calculate the relative risk associated with each food eaten, and finally provide a ranked table of foods eaten, showing relative risks.

Part2:

Identify the type of food poisoning, the common food ingredient (if any) which might have caused it, and briefly describe the circumstances under which the agent might have been introduced and the toxin developed in the food.

Aim of exercise:

i. To gain an insight as to the use of epidemiological risk analysis in identifying the most likely environmental source of disease from a number of potential co-sources.

ii. To use qualitative knowledge to support quantitative information generated by risk analysis, in order to reach sensible conclusions which can be applied to successful, early intervention.

Hints:

i. When confronted with analysis, always start by constructing tables with totals, in this case, one for each food type so that you can compare results from each table.

ii. Each item in the above table is a “mixed food”. Ultimately you need to know what ingredients are in each to carry out a proper investigation. Consult recipes.

iii. Identify the most likely food poisoning type from the symptoms, then identify likely sources and growth factors for the relevant food poisoning bacterium from the literature.

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