Two Series of Chart Data: Amount of Change or Percentage Growth?

First identify the two series

A paired bar chart compares two values for each category. Before comparing bar heights, we need the category labels, the two series labels and a common unit. This lesson supplies the underlying chart data explicitly in a table, so every calculation can be checked without estimating a drawing.

Three fictional centres issued the following numbers of practice booklets. Each column is one weekly series; every entry is a count of booklets.

Centre	Week 1 booklets	Week 2 booklets
P	20	40
Q	80	120
R	100	130

If these values are drawn as paired bars, each centre has one Week 1 bar and one Week 2 bar, using the same booklet scale. Do not compare visual heights from charts with different scales without reading their numbers.

Biggest increase can mean two different things

Centre P increased by 40 − 20 = 20 booklets. Its percentage growth is 20/20 × 100 = 100%: it doubled.

Centre Q increased by 120 − 80 = 40 booklets. Its percentage growth is 40/80 × 100 = 50%.

Therefore Q added more booklets, while P had greater percentage growth. There is no contradiction. The absolute increase counts extra booklets; percentage growth compares that increase with a different starting base for each centre.

Do not call P's result a 200% increase. Its final value is 200% of its initial value, but the extra part is 100% of the initial value.

Combine totals before finding total growth

Week 1 total = 20 + 80 + 100 = 200 booklets. Week 2 total = 40 + 120 + 130 = 290 booklets.

Combined growth = (290 − 200)/200 × 100 = 45%. Do not simply average the three centre growth percentages: their Week 1 bases are unequal. The existing averages lessons explain why unequal groups need their actual weights.

A Week 2 share uses 290 as the whole; a centre's growth uses that centre's Week 1 count. Writing the denominator's meaning beside the calculation prevents mixing these comparisons.

Check yourself

1. Find Centre R's absolute increase and percentage growth.
2. What percentage of Week 2's total came from Centre Q? Round to two decimal places.
3. Find the combined percentage growth from Week 1 to Week 2. Check it by adding the three increases first.

Worked answers

1. R's increase = 130 − 100 = 30 booklets. Growth = 30/100 × 100 = 30%. The final value is 130% of the original; the increase is 30%.
2. Q contributed 120 of 290 booklets. Share = 120/290 × 100 = 41.38%, rounded to two decimal places. The exact fraction of the total is 12/29. Using Q’s 80 would compare its final amount with its own starting amount; it would not be a share of the Week 2 total.
3. Total growth = (290 − 200)/200 × 100 = 45%. Independent check: increases are 20, 40 and 30; together 90. Then 90/200 × 100 = 45%. A 45% increase on 200 is 90, giving 290.

This is introductory practice with explicitly labelled chart data. It does not replace practice reading real graph axes and legends. The dataset and questions are original, not attributed to an SSC paper.