3.3 Observational studies

Observational studies (Fig. 3.2) are used to answer relational RQs. They are commonly used, and sometimes are the only possible study design that can be used. These are discussed further in Chap. 3.3.

Definition 3.2 (Observational study) In an observational study, researchers do not impose the comparison or connection upon those in the study to (potentially) change the response of the participants.

FIGURE 3.2: An observational study, used to answer a relational RQ

Definition 3.3 (Condition) Conditions: The conditions of interest that those in the observational study are exposed to.

Example 3.3 (Observational study) Consider again this RQ (Barrett et al. 2010):

Among Australian teens with a common cold, is the average duration of cold symptoms shorter for teens taking a daily dose of echinacea compared to teens taking no medication?

This would be a relational RQ if the researchers do not impose the echinacea (that is, the individuals make this decision themselves). For this RQ, the conditions would be taking echinacea, or not taking echinacea (Fig. 3.3).

FIGURE 3.3: Observational studies

Broadly speaking, three types of observational studies exist (Table 3.1):

• Retrospective: look into the past for the outcome;
• Prospective: look into the future for the outcome;
• Cross-sectional: obtain the outcome in the present.

TABLE 3.1: The three types of observational studies

Type	O	C	Reference
Retrospective	Now	Earlier	Sect. 3.3.1
Prospective	Later	Now	Sect. 3.3.2
Cross-sectional	Now	Now	Sect. 3.3.3

These differ in when the Outcome and Comparison/Connection are observed. Many specific types of observational studies exist (case-control studies; cohort studies; etc.), but we will not delve into these.

3.3.1 Retrospective studies

In retrospective studies, the Outcome (and response variable) is observed now, and the researchers look back to see what Comparison/Connection group (and explanatory variable) was in the past (e.g. case-control studies).

Example 3.4 (Retrospective studies) An Australian study (Pamphlett 2012) examined patients with and without sporadic motor neurone disease (SMND), and asked about past exposure to metals. The response (whether or not the respondent had SMND) is assessed now, and whether or to they had exposure to metals (explanatory) is assessed from the past. This is a retrospective observational study.

3.3.2 Prospective studies

In prospective studies, the Comparison/Connection (or explanatory variable) is determined now, and researchers look ahead to assess or measure the Outcome (or response) (e.g. Prospective cohort studies).

Example 3.5 (Prospective studies) A study (Choi and Curhan 2008) measured the softdrink consumption of men, and determined who experienced gout over the following 12 years. The response (whether or not the individuals experience gout) is determined in the future. The explanatory variable (the amount of softdrink consumed) is measured now. This is a prospective observational study.

3.3.3 Cross-sectional studies

In cross-sectional studies, both the Outcome (response) and Comparison/Connection (explanatory variable) are gathered now.

Example 3.6 (Cross-sectional studies) A study (J. Russell et al. 2014) asked older Australian their opinions of their own food security, and recorded their living arrangements. Individuals’ responses to both living arrangements and opinions on food security are obtained now. This is a cross-sectional observational study.

Think 3.1 (Cross-sectional studies) In South Australia in 1988–1989, 25 cases of legionella infections (an unusually high number) were investigated (O’Connor et al. 2007). All 25 cases were gardeners, with hanging baskets of ferns.

Researchers compared 25 cases with legionella infections with 75 non-cases, matching on the basis of age (within 5 years), sex, post codes. The use of potting mix in the previous four weeks was associated with an increase in the risk of contracting illness of about 4.7 times.

What type of observational study is this?

Retrospective: people were identified with an infection, and then the researchers looked back at past activities.