Knowledge Organiser: Bar Charts and Pictograms
Key Terms
- Bar chart: A chart using equal-width bars to compare categories
- Pictogram: A chart using symbols where each symbol represents a set number
- Key: States what each symbol in a pictogram represents
- Compound bar chart: Bars split to compare two or more groups side by side
- Discrete data: Separate categories (the data type bar charts represent)
- Frequency: The count for each category, shown on the vertical axis
Must-Know Facts
- All bars must be the same width with equal gaps between them
- The y-axis must start at zero: truncating it misleads the reader
- Every chart needs a title, axis labels, and a scale
- Pictogram symbols must all be identical in size
- Partial symbols (halves, quarters) must be drawn accurately, not estimated
- Bar charts are for discrete/categorical data, not continuous data
- A pictogram always requires a key showing the value of each symbol
Key Methods
- Reading pictogram: count symbols × value shown in key
- Drawing pictogram: value ÷ key value = number of symbols needed
- Reading bar chart: trace bar top horizontally to y-axis scale
- Choose y-axis scale so the tallest bar fills most of the available space
Common Mistakes
- Misreading the pictogram key: Each symbol represents the key value: if the key says 1 symbol = 4 people and there are 2.5 symbols, the frequency is 10, not 2.5
- Bars touching in a bar chart: Bars in a bar chart for categorical data should have gaps between them: touching bars indicate continuous data (histogram)
- Not labelling axes: Both axes must be labelled with a title and units: an unlabelled y-axis loses a mark even if the bars are correct
- Unequal bar widths: All bars in a bar chart must have equal widths: only histograms can have variable-width bars