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Evidence-Based Optometry

The rest of the scientific studies on the evidence pyramid are differentiated by their differing methodologies. Use the chart below and look at the Methods section of an article to identify the type of study design. 


Did an experiment take place?

Studies can be experimental or observational. In other words, was an experiment conducted? Or did the author simply observe features of patients' conditions? 

Experimental studies are called controlled trials. A controlled trial tests treatments using two groups. One group is assigned a treatment (such as a drug) while the other is assigned either a different treatment or placebo treatment.


Was the study randomized?

Controlled trials can either be randomized or non-randomized.

Randomization refers to how patients are put into groups. Randomized studies put patients into groups using a random method like a computer generator or the flip of a coin.


Was there a control group?

Observational studies either have a control group (a group without the disease or symptoms being examined) for comparison or do not.

A case study looks at the symptoms of one patient (or the symptoms of a small group of patients) so there is no comparison group.

What was the study's relationship to time?

  • cohort study looks forward in time; for example, following a group of patients with cataracts over 10 years to see if surgery for cataracts decreased mortality incidence.

Real World Example: Predicting Vision-Related Disability in Glaucoma

  • case-control study looks backward in time; for example, examining a group of patients with lung cancer and looking back in their medical records to see how many were smokers.

Real World Example: Social, health and ocular factors associated with primary open-angle glaucoma amongst Chinese Singaporeans

  • cross-sectional study examines one period in time; for example, collecting data from a group of patients with allergic conjunctivitis and recording the length and time of their allergy episodes.

Real World Example: Relationship Between Open-angle Glaucoma and Stroke: A 2010 to 2012 Korea National Health and Nutrition Examination

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