Biology 4a assignment

N O N - E X P E R I M E N T A L S T U D Y D E S I G N S P A R T 1 : C R O S S - S E C T I O N A L S T U D Y D E S I G N

6310-WEEK 4

LEARNING OBJECTIVES

• Describe different types of non- experimental study designs

• Identify the strengths and weaknesses of non- experimental study designs

• Critically appraise the strength of evidence in non- experimental study designs

• Utilize major national health surveys to extract local and regional data

NON-EXPERIMENTAL STUDY DESIGNS

• Non-experimental or observational study design: Research design in which the investigators simply observe subjects without making any interventions.

• Goals • Descriptive: examine the distribution of predictors and

outcomes in a population • Analytic: examine associations between predictor and

outcome variables

MAIN NON-EXPERIMENTAL STUDY DESIGNS

• Cross-sectional study design • Cohort study design • Case control study design

• Other types • Case reports • Case series • Natural history studies • Ecological studies

CROSS-SECTIONAL STUDY DESIGN

CROSS-SECTIONAL STUDY DESIGN

• Time frame: • All measurements for each subject are taken at the same

time. • No follow-up period.

• Goals • Describe variables and their distribution patterns within a

sample. • Estimate prevalence (the proportion who have a disease or

condition at one point in time) • Examine associations but not the direction of the

relationship.

CROSS-SECTIONAL STUDY DESIGN

Defined Population

Exposed Have Disease

Exposed Do not Have

Disease

Not Exposed Have Disease

Not Exposed Do not Have

Disease

Collect data on exposure and disease/condition

Four groups are possible

STATISTICAL MEASURES IN CROSS-SECTIONAL DESIGNS

• Prevalence: the proportion who have a disease or condition at one point in time

• Prevalent cases are existing cases at a point in time. This is in contrast to incident cases which are new cases over a period of time.

• Prevalence = #of people with health outcome # of people in study population

• Example: Using self-reported information from a sample of U.S. adults in 2017, the Centers for Disease Control and Prevention estimates obesity prevalence in Texas at 33%. • For obesity prevalence by state, click here.

ADVANTAGES OF CROSS-SECTIONAL STUDIES

• Provide descriptive information about prevalence (the proportion who have a disease or condition at one point in time), a unique feature of cross- sectional studies

• Short duration, inexpensive, no problem with loss to follow-up

• Good first step for a cohort or experimental study providing baseline characteristics and/or revealing cross-sectional associations of interest

DISADVANTAGES OF CROSS- SECTIONAL STUDIES

• Difficult to establish causal relationships • Not practical for rare conditions (need large sample

size) • Potential survivor bias (no information about

patients who died) • Potential bias in measuring predictors

SERIAL CROSS-SECTIONAL SURVEYS

• A series of cross-sectional studies in the same population

• Used to draw inferences about changing patterns over time

• Longitudinal time frame • Different from cohort studies because a new sample is

drawn each time

EXAMPLES CROSS-SECT IONAL STUDIES

NATIONAL HEALTH AND NUTRITION EXAMINATION SURVEY (NHANES)

• NHANES is “a program of studies designed to assess the health and nutritional status of adults and children in the US.”

• It collects data through a combination of interviews and physical examinations. • The survey interview includes demographic, socioeconomic,

dietary, and health-related questions. • The examination component consists of medical, dental, and

physiological measurements, as well as laboratory tests. • Findings provide prevalence estimates of chronic

conditions and assess associations between nutritional status and health promotion and disease prevention.

• For more information about NHANES, visit their website available here.

BEHAVIORAL RISK FACTOR SURVEILLANCE SYSTEM (BRFSS)

• BRFSS is a serial cross-sectional telephone survey designed to monitor state-level prevalence of the major behavioral risks among adults associated with premature morbidity and mortality.

• It is initiated and supported by the Centers for Disease Control and Prevention (CDC) and conducted by state health departments.

• Findings support planning and evaluating health promotion and disease prevention programs.

• For more information about BRFSS, visit their website available here.