Cross Sectional Study

Epidemiological research is a type of medical research that is specialized in determining the distribution of diseases in population, risk factors and health determinants relating to diseases.

There are 2 types of Epidemiological research:

Observational studies:

In these studies, no intervention from investigator as we observe effect of something like risk factor or treatment without attempting to change who is exposed to it or not.

These studies are classified into:

Individual based:

a)      Descriptive studies: are to describe the occurrence of the disease in population (pattern of disease) such as case reports or series.

b)     Analytical studies: are to analyze the relationship between exposure and outcome such as Cross sectional, Case control, Cohort study.

Population Based:

a)      Descriptive: includes health survey.

b)     Analytical: includes ecological study.

Experimental studies:

They are interventions to change many variables relating to disease such as risk factors, disease determinant, exposure, progress through treatment, ……etc such as Randomized: control or clinical trial, Non-Randomized: Quasi-Experimental field trial or Community Trial.

We will talk about Cross sectional Study in this post.

Imagine that you are watching the TV and are turning the channels when your eyes fall on breaking news saying that HIV spreads in the west of America by 20%. And after you saw this ratio, you entered a mode of overthinking that you were wondering how this ratio was estimated.

So, the answer for this question is the cross-sectional study.

Cross sectional study is a type of observational study design that assesses the prevalence of diseases in a certain area at a point in time. And it is related to prevalence ratio (20% as you see in TV). 
The prevalence ratio is (No. of people getting that disease/ No. of people undergoing this survey).

To understand more, let suppose that WHO sends a team to village in west of Africa for example to determine the prevalence of Lung cancer there.

So, the team begin to take a sample from the village and the total people undergoing the survey is 300 people and the team examine the people. 
They found that 150 people had lung cancer So, the prevalence ratio = (No. of people getting that disease/ No. of people undergoing this survey) = 150/300 = 0.5.

This is way is related to descriptive cross-sectional study only as descriptive study assess only the prevalence of health event or diseases in the population.

The second way is related to analytical cross-sectional study that measures the association between exposure and outcome as they are assessed simultaneously.

And here, the way to determine prevalence ratio is different as well as determination of prevalence odds ratio.  

To understand the previous terms, let imagine that Egypt for example wants to know the association with obesity and diabetes in Cairo, So the Ministry of health sent a team to estimate the prevalence ratio and odds ratio (measures the strength of association between events).

So, they take a sample of about 200 peoples from all over of Cairo.

Therefore: they have to make table 2*2 to determine the relation between obesity and diabetes.

	Diabetes Yes (outcome)	Diabetes NO
Obesity YES (exposed)	80  (A)	20 (B)
Obesity NO	40  (C)	60  (D)

After they collected the previous data, they estimated the following:

Prevalence of diabetes in Cairo = people with diabetes (A+C) / total people in survey (A+B+C+D) = 120/200 = 0.6

Prevalence of diabetes among people with obesity = diabetes with obesity (YES) (A) / total people with obesity (A+B) = 80 / (80+20) = 0.8

Prevalence of diabetes among people without obesity = diabetes without obesity (C) / total people without obesity (C+D) = 40 / 40+60 = 0.4

So, the prevalence ratio = Prevalence of diabetes among people with obesity / Prevalence of diabetes among people without obesity = 0.8 / 0.4 = 2

Therefore, ratio of people with diabetes is twice greater when people have obesity.

To calculate the Prevalence odds ratio, you have to make cross multiply = A*D / B*C = 80*60/20*40 = 6

So, odds that diabetes people would be obesity appear to be 6 times odds that non-diabetes people would be obesity. Furthermore, there is strong relationship between diabetes and obesity.

Advantages of Cross-sectional study:

1)     Good for describing the magnitude and distribution of health problems.

2)     Generalizable results if population-based sample

3)     Quick, conducted over short period of time, easy, inexpensive.

4)     Can study multiple exposures and disease outcomes simultaneously.

Disadvantages of Cross-sectional study:

1)     Doesn’t determine the cause and effect.

2)     Not good for rare diseases or exposures.

3)     Can’t calculate the risk.

4)     Temporal relationship difficult to be established.

5)     Are subject to recall bias.

6)     Can study only one disease at a point of time.