1.3 Observational studies
Observational studies (Fig. 1.2) are used to answer relational RQs. They are commonly used, and sometimes are the only possible study design that can be used. We discuss these further in this chapter.
Figure 1.2: An observational study, used to answer a relational RQ
Example 1.3 (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. 1.3).
Figure 1.3: Observational studies
Broadly speaking, three types of observational studies exist (Table 1.1):
- Retrospective: look into the past for the comparison;
- Prospective: look into the future for the outcome;
- Cross-sectional: obtain the outcome in the present.
| Type | O | C | Reference |
|---|---|---|---|
| Retrospective | Now | Earlier | Sect. 1.3.1 |
| Prospective | Later | Now | Sect. 1.3.2 |
| Cross-sectional | Now | Now | Sect. 1.3.3 |
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.
1.3.1 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).
1.3.2 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).
1.3.3 Cross-sectional studies
In cross-sectional studies, both the Outcome (response) and Comparison/Connection (explanatory variable) are gathered now.
Lemma 1.1 (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?