Overview
In data science and analysis, the independent variable is the factor you control or observe changes in. Common examples include time, categories like regions, or measurable inputs like temperature. The dependent variable is the result or outcome that changes in response to the independent variable.
In this area chart showing electricity sources, the year is the independent axis, since it is the factor that is observed. The dependent variable is electrical generation (measured in thousand megawatt-hours) because it is the outcome that changes in response to the change in time.
Placement on Chart Axes
Independent and dependent variables are often represented as dimensions in charts, where they are plotted on different axes to show their relationship. The appropriate placement of independent and dependent variables depends on the type of chart:
- In most conventional charts, such as column, line, and area charts, the independent variable is typically placed along the horizontal axis (x-axis), and the dependent variable is placed along the vertical axis (y-axis).
- In charts where the orientation of the axes is rotated, such as horizontal bar charts and arrow plots, the independent variable is placed along the vertical axis (y-axis), and the dependent variable is placed along the horizontal axis (x-axis). These charts are useful for displaying categorical data with long category labels or when the focus is on comparing relative magnitudes across categories.
Sometimes, there is no clear-cut independent and dependent variable in a chart. For example, in a scatterplot comparing weight and height, neither variable may be strictly dependent on the other. In such cases, both variables could be plotted on either axis.
In scientific experiments, other factors that might influence the dependent variable are often "controlled" (held constant) to isolate the true effect of the independent variable. These are referred to as control variables.
In most conventional charts, such as the column chart on the left, the independent variable (country) is placed on the horizontal axis and the dependent variable (total area) is placed on the vertical axis. In the stacked bar chart on the right, the orientation of the chart axes has been rotated to create more space for category labels. As a result, the independent variable appears on the vertical axis and the dependent is on the horizontal axis.
Examples
Understanding the relationship between independent and dependent variables, as well as the potential role of control variables, becomes clearer with practical examples:
Education—Class Size vs. Test Scores: A teacher examines whether the number of students in a class affects test performance. The independent variable is the number of students, while the dependent variable is the test score. Control variables may include the subject being taught and teacher's qualifications.
Health Science—Exercise Duration vs. Heart Rate: A researcher investigates how the length of exercise sessions affects heart rate. The independent variable is the duration of exercise, while the dependent variable is the heart rate in beats per minute. Controlled factors include the participant’s age, fitness level, and the type of exercise performed.
Marketing—Ad Placement vs. Click Rates: Marketers test whether the location of an ad in a search results page impacts user interaction. The independent variable is the placement of the ad, while the dependent variable is the click-through rate. Control variables include the ad content and the audience demographics.
Sports Science—Practice Hours vs. Win Percentage: Coaches assess whether more practice hours lead to improved team performance. The independent variable is the number of practice hours per week, while the dependent variable is the win percentage over a season. Control variables include the players’ skill levels and the quality of opposing teams.
Technology—Screen Brightness vs. Battery Life: Developers evaluate the impact of screen brightness settings on smartphone battery duration. The independent variable is the brightness level, while the dependent variable is the battery life in hours. Controlled factors include the phone model and the applications running during the test.
