Medium4 marksStructured
Data Visualization and RepresentationCumulative FrequencyMedianInterquartile RangeQuartiles

AQA GCSE · Question 09.2 · Data Visualization and Representation

0 100 200 300 400 500 Cumulative Frequency 0 1 2 3 4 5 Number of rewards 250 125 375

The tech company wants to change the reward system so that the median number of rewards is increased and the interquartile range is increased. They make some changes and take a new sample of 500 customers. The cumulative frequency graph for the new data is shown. Use the graph to find the median and interquartile range for the new sample.

How to approach this question

1. **Find the Median**: * The total frequency is 500. The median position is at 50% of the total, so 0.5 * 500 = 250. * Find 250 on the cumulative frequency (y-axis). * Draw a horizontal line from 250 to the curve. * Draw a vertical line from that point down to the number of rewards (x-axis). Read the value. 2. **Find the Lower Quartile (Q1)**: * The Q1 position is at 25% of the total, so 0.25 * 500 = 125. * Find 125 on the y-axis, go across to the curve, and then down to the x-axis. Read the value. 3. **Find the Upper Quartile (Q3)**: * The Q3 position is at 75% of the total, so 0.75 * 500 = 375. * Find 375 on the y-axis, go across to the curve, and then down to the x-axis. Read the value. 4. **Calculate the Interquartile Range (IQR)**: * Subtract the lower quartile value from the upper quartile value (IQR = Q3 - Q1).

Full Answer

To find the median and quartiles from the cumulative frequency graph of 500 customers: - **Median (Q2)**: Find the value at the 500/2 = 250th position. - Go to 250 on the y-axis, across to the curve, and down to the x-axis. This reads as **2 rewards**. - **Lower Quartile (Q1)**: Find the value at the 500/4 = 125th position. - Go to 125 on the y-axis, across to the curve, and down to the x-axis. This reads as **1 reward**. - **Upper Quartile (UQ)**: Find the value at the 3*500/4 = 375th position. - Go to 375 on the y-axis, across to the curve, and down to the x-axis. This reads as **3 rewards**. - **Interquartile Range (IQR)**: IQR = UQ - Q1 = 3 - 1 = **2**. So, for the new sample: - **Median = 2** - **Interquartile Range = 2**
This question requires reading values from a cumulative frequency graph. The total frequency (the maximum value on the y-axis) is 500. 1. **Median (Q2)**: The median is found at the 50% point of the data. * Position = 0.5 × 500 = 250. * On the graph, locating 250 on the vertical axis, moving horizontally to the curve, and then vertically down to the horizontal axis gives a value of **2 rewards**. 2. **Lower Quartile (Q1)**: The lower quartile is found at the 25% point. * Position = 0.25 × 500 = 125. * On the graph, locating 125 on the vertical axis, moving across to the curve, and down to the horizontal axis gives a value of **1 reward**. 3. **Upper Quartile (Q3)**: The upper quartile is found at the 75% point. * Position = 0.75 × 500 = 375. * On the graph, locating 375 on the vertical axis, moving across to the curve, and down to the horizontal axis gives a value of **3 rewards**. 4. **Interquartile Range (IQR)**: The IQR is the difference between the upper and lower quartiles. * IQR = Q3 - Q1 = 3 - 1 = **2**.

Common mistakes

✗ Using (n+1)/2 instead of n/2 for finding positions on a CF graph (this is for raw discrete data). ✗ Misreading the scales on the axes. ✗ Calculating the range instead of the interquartile range.

Practice the full AQA GCSE Statistics Higher Tier Paper 1

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