Research Design:Choosing the Study Design

Kimberly R. Barber

Director of Research

McLaren Regional Medical Center

Yes No

Yes No Yes No

Exposure Outcome Exposure & Outcome Simulteneous

Exposure Outcome

Assign Exposure?

Experimental Observational

Random allocation Comparison group

RCT Non-RCT

Analyticstudy

Descripstudy

Direction?

Cohort Case-Control

Cross-Section

Design to Employ

Dependent on the hypothesis posed.– Is your intent to observe, associate factors, or

show cause and effect?– Are exposure or outcome factors common or

rare?– Are your resources many or constrained?

Types of Study Designs

Observational Designs– Descriptive– Analytic

Cross-sectional, Case-control, Cohort

Experimental Designs– Quasi experimental

Non randomized or non control trial

– True experimental Randomized Controlled Trial

Descriptive Studies

No assignment of exposure or risk factor. Intent is to observe and record.

Record events or activities.– Single event or case = Case Report.– Several events or cases = Case Series.

Cross-Sectional Studies

Measurement of risk and outcome at the same time.

Outcome

Risk factor

Cross-Sectional Design

The only study capable of calculating prevalence.

– Proportion of the population with the outcome at any point in time.

Cross-Sectional Studies (cont)

Most useful if exposure continues right up to time that outcome is recognized.

Often conducted as screening and classification preamble to prospective studies.

Application Exercise: Cross-Sectional Study

Design a cross-sectional study that examines the relationship between dietary sodium and hypertension in middle-aged men.

What research question can we answer?

Cross-Sectional StudiesAdvantages

Cheap and quick studies. Data is frequently available through current

records or statistics. Ideal for generating new hypothesis.

Cross-Sectional StudiesDisadvantages

The importance of the relationship between the cause and the effect cannot be determined.

Temporal weakness:– Cannot determine if cause preceded the effect or

the effect was responsible for the cause.– The rules of contributory cause cannot be fulfilled.

Case-Control Studies (Retrospective)

Case/Control Studies.– Group subjects according to the presence or

absence of the outcome.

– Review past histories of the subjects for the occurrence of suspected risk factors.

?

?

DiseaseRisk

Case / Control Studies (cont)

Case / Control studies have two main purposes:

Descriptive – Describe the risk factor profile for an outcome.

Analytic – Analyze associations between outcome and risk

factors.

Application Exercise:Case / Control Study

Describe a case/control study on the relationship between childhood obesity, smoking history, and occurrence of hypertension in middle-aged men.

What research question can we answer?

Case / Control StudiesAdvantages

Are good initial explanatory studies.– Investigators can explore multiple risk factors

simultaneously for one outcome.

Are efficient, relatively cheap, and quick.– Data available through

chart review.

Case / Control Studies Advantages (cont)

Well suited for rare diseases.– Since the study begins with subjects who already

have the outcome, it is easier to accumulate enough subjects for significant results.

Case / Control StudiesDisadvantages

Data Quality– Data with inadequate detail, questionable

reliability, or use a different standard to judge disease severity.

Recall bias– Subjects who have unpleasant experiences may

recall past differently than control subjects.

Retrospective StudiesDisadvantages (cont)

Sampling bias– Sample usually not representative of all subjects

who could be included.

Other– Capable of studying only one outcome at a time– Cannot calculate prevalence or incidence.– Subject to confounding factors. – Cannot prove contributory cause.

Cohort Studies(Prospective)

Subjects identified according to the presence or absence of the risk factor.

Followed over time until the outcome occurs or becomes evident.

Cohort Studies (cont) Subjects with and without the suspected risk

factor are followed for the development of the outcome.

– The frequency of the outcome is compared between the two groups.

Disease

?

?

Risk

Cohort Studies (cont)

Cohort studies have two main purposes: Descriptive

– Describe the incidence of outcome over time.

Analytic – Analyze associations between risk factors and

outcome.

Historical Cohort Studies(Non concurrent)

The following all happen in the past:– Assembly of the cohort according to the presence

of absence of risk factors.– Baseline measurements– Follow-up and outcomes

Exposure

?

?

Disease

Historical Cohort (cont)

Are not retrospective studies.– Subjects are grouped according to the presence

or absence of risk factors (past or present).

– Subjects in retrospective studies are grouped according to the presence or absence of the outcome.

Cohort StudiesAdvantages

More powerful design for defining incidence. Powerful design for associating the cause

with the effect.– Can suggest that the cause precedes the effect.– Data can be collected in a comprehensible and

uniform fashion. Exception: historical cohort with same measurement

weaknesses as the case/control.

Cohort StudiesAdvantages (cont)

No recall bias. Cohort designs can examine many outcomes

for potential risk factors under investigation.

Cohort StudiesDisadvantages

Expensive in time, money, and number of patients necessary to demonstrate significant differences between groups.

Less likely than retrospective studies to uncover new risk factors.

Also subject to confounding.

Cohort StudiesDisadvantages (cont)

Loss of valuable information due to patient attrition.

Patients may change their behaviors or risk factors after the initial grouping of subjects resulting in misclassification and study error.

Clinical Trial

Experimental study– Unique features: – Intervention in the subject’s care for the

purposes of the study.– Randomization of subjects. – Control group comparison.

Placebo or treatment

Clinical Trial Design

Randomized

Intervention/

Effect

Outcome

X

X

Clinical Trial (cont)

Randomization– Subjects are randomly assigned to control or

experimental group.

– Groups are similar in every way except for the intervention under study.

– Each subject has equal probability of being placed in either group.

Clinical TrialsAdvantages

Subject to the fewest methodological biases of all study designs.

Most powerful study designed to show contributory cause.

Clinical TrialsDisadvantages

Is the most expensive study design in terms of money, time, and number of patients.– Issues of patient attrition and compliance may

invalidate the results.– Can be problematic for ethical reasons.

Use of placebo Harm outweighing benefits Zero tolerance for some exposures

Choosing the Specific Design

Study design is highly dependent on the type of analysis. (3)

Type of analysis is dependent on the hypothesis posed. (2)

The hypothesis is dependent on the intent of your research. (1)

1. Research Study Intent

Know the problem.

Determine what you want to conclude.

Formulate the question.

Examples of Intent

I intend to show that aspirin resistance is associated with the severity of heart disease. – I will compare levels of aspirin resistance among

patients with differing severity of heart disease. I intend to show that breast feeding is

protective against allergies developing in infants. – I will compare rates of allergies among women

who breast feed and those who do not.

2. Research Hypothesis Know the question you

want answered.

Restate the question into terms of Ho and Ha.

Think about corresponding analysis.

Examples of Hypotheses

Is aspirin resistance associated with heart disease?– Aspirin resistance increases the risk of heart

attack.

Is breast feeding associated with allergies?– Breast feeding decreases the risk of allergies in

babies.

3. Statistical Plan of Analysis

Correlation?

Comparison?

Association?

Difference?

Examples of Analysis

The level of aspirin resistance is compared between those with heart attack and those without.– Differences in resistance scores between two

groups (Student T-test)

The rate of infant allergies is compared among women who breast feed and those who do not.– Relative risk association (Chi square analysis)

Choose Appropriate Design

Cross-sectional.

Case-control.

Cohort.

Clinical Trial.

Apply the Best Design

Think about the measures to be used.

Know the analysis required.

Rethink desired conclusions.

Summary

Study Design key to determining cause or just association.

Each design has unique strengths & weaknesses.

The design drives the conclusions.