A histogram is a type of mathematical chart where data is represented by bars. Each bar shows you how many items fall into a particular category or class. An example of class is "IQ Scores between 100 and 110" or "People who voted Democrat in the 2000 election," Histograms give you an idea of what kind of shape the data fits (for example, normal distribution or chi-squared) and how the data is skewed (left, right, or centered). How many items are represented by a bar is that bar's "frequency."
Locate the bar for which you want to find the frequency. For example, if you want to know how many people have an IQ of between 100 and 110, locate the bar centered between 100 and 110 on the horizontal number line.
Read where the top of the bar falls on the vertical number line (the vertical axis). This is the frequency for that bar. Use a ruler, placed level with the top of the bar, to help you see where the bar falls on the vertical axis.
Repeat Steps 1 and 2 for every bar in the chart and add them together to find the cumulative frequency for that chart. For example, if you have bars with 10, 11 and 12 as individual frequency counts, then 10+11+12=33.
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- If your histogram is on graph paper, you can count up the number of squares on the graph paper instead of reading where the top lies on the vertical axis.
- Make sure you know if you are looking for the relative frequency (single bars), or cumulative frequency (all the bars added together). These terms are often confused.
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