E9.1 Classifying statistical data

Learning objectives

E9.1.1Classify and tabulate statistical data• Classify and tabulate statistical data. e.g. tally tables, two-way tables.

Classifying data means placing every observation into exactly one suitable category, then tabulating the counts so they can be checked and compared.

Choose categories that do not overlap and together include every possible observation. Record each observation once: omission and double-counting both change the frequencies.

Table	Use	Recording rule
Tally table	one variable has several categories	add one tally per observation; bundle the fifth tally across the previous four, then write the numerical frequency
Two-way table	each observation has one category from each of two variables	place it in the single cell where its row and column categories meet

For shirt colours blue, red, blue, green, red, blue, the frequencies are blue $3$ , red $2$ , green $1$ . Their sum is $6$ , matching the six observations; tally marks are the counting record, not the final numerical frequencies.

	Part-time	Full-time	Total
Lecturer	8	12	20
Professor	3	7	10
Total	11	19	30

Add across each row and down each column. Both sets of totals must give the same grand total. Find a missing cell by subtracting the other known cells from its correct row or column total, not from an unrelated total.

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