Learn histograms
A histogram displays the distribution of numerical data in adjoining intervals called bins. Bar height represents frequency, while the overall pattern can reveal shape, center, spread, gaps, and unusual regions.
You will read histogram intervals, compare bar heights, and describe a distribution.
The idea in plain English
A histogram groups numerical values into intervals. Each bar shows how many values fall inside one interval.
The bars touch because the number intervals continue without category gaps.
How to recognise this kind of question
- The horizontal axis shows number ranges rather than named categories.
- Bars usually touch.
- The vertical axis shows frequency or count.
Quick decision: Find the correct interval first, then read that bar's height.
The method
- Read the horizontal axis to identify each bin and its boundaries.
- Use bar height and the frequency scale to determine how many observations fall in each interval.
- Study the full pattern before describing shape, approximate center, spread, gaps, or unusual regions.
- Remember that grouped bars do not reveal every exact raw value.
Worked examples
A histogram's 20–30 bin has height 8 and its 30–40 bin has height 5.
- The first bar represents 8 observations from 20 up to 30.
- The next bar represents 5 observations from 30 up to 40.
- Add the frequencies when the question asks about the full 20–40 region.
Answer: 13 observations from 20 up to 40
Bars for 0–9, 10–19, and 20–29 have heights 2, 6, and 3. Which interval is most common?
- Compare the three bar heights.
- The tallest bar has height 6.
Answer: 10–19
Your turn—with help
The 30–39 bar has height 7. What does that mean?
Need a hint?
Connect the interval to the bar's count.
Show the answer and steps
- The horizontal interval is 30–39.
- The vertical height is a frequency of 7.
Answer: Seven values are from 30 through 39.
Watch out for this mistake
Incorrect: A bar from 10–19 with height 5 means the total is 15
Why it fails: The height is a count, not a number to add to the interval boundary.
Do this instead: It means five data values fall from 10 through 19.
Quick check
Try these without looking. Then open each answer to check your thinking.
A 5–9 bar has height 4. How many values are in that interval?
Need a hint?
Read the height.
Show the answer and steps
- The bar height is the frequency.
Answer: 4
Which interval has the greatest frequency?
Need a hint?
Look for the tallest bar.
Show the answer and steps
- Compare heights, not widths.
Answer: The interval with the tallest bar
Can a histogram identify every exact data value?
Need a hint?
Values are grouped into intervals.
Show the answer and steps
- The graph shows interval counts, not each original value.
Answer: No
Common mistakes
- Treating the horizontal scale as categories instead of numerical intervals.
- Leaving gaps between histogram bars when the bins are contiguous.
- Claiming an exact mean, median, range, or outlier from grouped data when only an estimate is supported.
- Confusing frequency with cumulative or relative frequency.
Practice tips
- State interval boundaries clearly, such as 20 up to but not including 30.
- Check totals before calculating percentages or cumulative frequencies.
- Compare a histogram's tail lengths when deciding whether it is skewed.
- Unlike a bar graph, a histogram groups numerical values into touching intervals.
Frequency and bins
Each bin covers a numerical interval. Its bar height is the number of observations in that interval. Add neighboring bar heights for combined or cumulative counts, and divide by the total for relative frequency.
Distribution shape
A roughly mirrored pattern is approximately symmetric. A longer left tail is skewed left, while a longer right tail is skewed right. Similar bar heights across the range suggest an approximately uniform distribution.
Center and spread
A histogram supports estimates of a typical value and the represented range, but grouping usually prevents recovery of the exact raw-data mean, median, or individual outliers.
Histogram vs. bar graph
Histograms display numerical intervals in order, so adjoining bins touch. Bar graphs compare separate categories and commonly leave space between bars.