C9.1 Classifying statistical data
- Syllabus
- 0580–2028–2029
- Topic
- C9.1
- Level
- Core
Learning objectives
Classify data in tally and two-way tables
Classifying data means placing each observation into a clearly defined category; tabulating data records the resulting counts in a structure that can be checked.
Choose categories that do not overlap and that cover every possible observation. Read each observation once, place it in exactly one category, and keep an 'other' category only when the question permits one.
| Table | Use | Recording rule |
|---|---|---|
| tally table | one variable has several categories | add one tally per observation, crossing every fifth tally through the previous four; write the numerical count as the frequency |
| two-way table | every observation has one category from each of two variables | place it in the single interior cell where its row and column categories meet |
For shirt colours blue, red, blue, green, red, blue, the tallies give blue 3, red 2 and green 1. The frequency column contains 3,2,1—not the tally symbols themselves. Their sum is 6, matching the six observations.
| Year group / travel | Walk | Cycle | Total |
|---|---|---|---|
| Year 1 | 6 | 4 | 10 |
| Year 2 | 7 | 3 | 10 |
| Total | 13 | 7 | 20 |
In a two-way table, each row total is the sum across its cells and each column total is the sum down its cells. The sum of all row totals and the sum of all column totals must both equal the grand total. Use subtraction from a known total to find a missing cell only after identifying the correct row or column.
A table organises and counts the data; interpreting patterns, comparing datasets, calculating averages or drawing statistical charts belongs to later topics. Never count one observation in two interior cells.