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How to Analyze Longitudinal Data

Instructor: Meredith Fontana
Longitudinal data involves taking measurements of specific information from a select group of people at different points over a period of time. Gathering the same type of data from the same group from different time periods allows us to see if changes took place and how much change occurred.

What Is Longitudinal Data?

Let's say that you're a medical researcher for a pharmaceutical company and you're wanting to study the long-term changes of a select group of patients who suffer from high blood pressure when given a new type of medication. How would you know if this medication benefited those in the group over different periods of time? One approach that may help you answer this question would be to conduct a longitudinal data study. When we do a longitudinal study, we focus on a select group and measure specific information at different points over a period of time.

So, in our blood pressure example, that would involve measuring each individual's blood pressure in the group prior to them taking the medicine or a placebo, and then measuring their blood pressure at different periods of time while the patients take the medication. Once you've collected several measurements to determine the effects of the medicine, the blood pressure values can then be used to analyze the effectiveness of the medication compared to those who took the placebo, as well as those who have a normal blood pressure range without medication.

Analyzing a Longitudinal Data Graph

Let's now look at the graph of a longitudinal data study and go through the steps of how to analyze the information it presents.

Longitudinal Data Graph
Longitudinal Data Graph

Setup of the Graph

Notice in this graph that we're comparing the percentage of men to women who work within three different professions from two different time periods (1970 and 2007). Before analyzing the information, let's first review how the graph is setup and what information is presented on it.

First, we see that on the horizontal axis the three different professions are listed (dentist, doctor, and lawyer). Also, notice that within each profession we have two different time periods (1970 and 2007) and two different colored bars (the blue bar represents men and the orange bar represents women). Next, we see that on the vertical axis we have the numbers from 0 to 100; these numbers represent the percentage range within each profession.

Numbers on the Graph

Now that we have an understanding of what the graph is going to present to us, let's look at the actual values. When looking at the value of men and women who were dentists in 1970, we see that 99% were men, while 1% were female. In 2007, the numbers for women look better, with men representing 71.8% of all dentists and women representing 28.2%.

Moving over to the doctors, we see that 8% of doctors in 1970 were women, while 92% were men. Thirty-seven years later, 30% of all doctors were female and 70% were men. Lastly, in 1970, 95% of all lawyers were men and 5% were women. In 2007, 67.4% were men, while 32.6% were women.

What Does this Study Show?

Because this study involves multiple careers over two different time periods comparing men with women, there are different pieces of information we can turn our attention to to analyze. For instance, we can compare the percentage of women who were dentists, doctors, and lawyers in 1970 with one another. Of the three careers, what one had the least amount of women in the field? What one had the most women in the field?

Based on the graph, dentistry had the least amount of women working in the field in 1970 at 1%, while doctors had the most at 8%. We can then compare the percentage of women in the professions from 2007. What field had the least amount of women? What one had the most? Again, dentistry had the least amount of women working at 28.2%, while lawyers had the most at 32.6%.

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