Data and tables

Facts that we can collect

Data is a set of facts that we collect: heights, colours, marks, temperatures, counts. On their own, facts are just a pile. Statistics is what we do to make sense of the pile. We organise it, show it in a picture, and say what it tells us.

Data is all around us. A cricket scorecard is data. So is the weather report. A school register, a bus timetable and the prices in a shop are data too.

The data table

We keep data in a table.

• Each row is one case: one student, one day, one match.
• Each column is one fact about every case. It is called a variable, because it can be different for each case.

Every cell holds one fact. So a table with 4 rows and 5 columns holds $4 \times 5 = 20$ facts.

You should find that one more row adds as many facts as there are columns. One more column adds a fact for every row.

Try it in Python

This table holds the four students of the picture. The program counts its rows and columns.

Where does data come from?

• Asking, in a survey: "What is your favourite colour?"
• Measuring: a height with a tape, a time with a stopwatch.
• Counting: the number of cars that pass a gate in one minute.
• Looking up: the weather records of a city.

If we cannot ask everyone, we ask a smaller group, called a sample. The last lesson of this course shows how to choose it fairly.

What makes a good table?

• One fact in each cell. Do not write "152 cm, blue" in one cell.
• The same unit in a column. Do not mix centimetres and metres.
• A clear mark for a missing value, such as a blank or the letters NA.
• A short name at the top of each column, with its unit, such as "height (cm)".

Never write $0$ for "we do not know". A zero is a real value, and it pulls the average down. The second practice below shows by how much.

Practise with new numbers

Common mistakes

• Mixing up rows and columns. A row is a case. A column is a variable.
• Writing zero for a missing value. Zero is a number, and it changes the average.
• Mixing units in a column. A column of heights must all be in the same unit.
• Putting two facts in one cell. A cell with "152 cm, blue" cannot be counted or sorted.

Remember these

• Data is a set of facts. A table keeps it.
• A row is one case. A column is one variable.
• Rows times columns is the number of facts.
• Mark a missing value clearly, never with a zero.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A table has 32 students and 5 columns. How many facts does it hold?

2. What is one column of a table called?

3. A table records the rainfall of a town on each day of June. How many rows does it have?

4. A student's height was not measured, and the table shows $0$ for it. Why is this a problem?