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By the end of this topic, you should be able to:
Statistical data is information that we collect about people, objects, or events. This could be anything from the ages of students in a class, to the colors of cars in a car park, to the test scores in a maths exam.
When we first collect data, it is usually messy and unorganized. We call this raw data. To make sense of raw data, we need to classify (organize it into groups) and tabulate (put it into tables).
A tally is a simple way of keeping count. Instead of writing numbers over and over, we draw short vertical lines called tally marks.
Representing 8 using tally marks:
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(One group of 5, then 3 more individual marks)
Representing 14 using tally marks:
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(Two complete groups of 5, then 4 more individual marks)
Why group in fives? Grouping in fives makes it much faster to count large numbers. Instead of counting every single mark, we can count in fives: 5, 10, 15, 20...
A frequency table is a table that shows how many times each value appears in a data set. The word frequency simply means "how many times" or "how often" something occurs.
Follow these steps:
Step 1: List all the different values that appear in your data
Step 2: Go through the data one item at a time, making a tally mark next to the correct value
Step 3: Count up the tally marks for each value to find the frequency
Step 4: Write the frequency number in the final column
Raw data: 5, 4, 5, 5, 4, 2, 2, 1, 3
Let's organize this into a frequency table:
| Age | Tally | Frequency |
|---|---|---|
| 1 | | | 1 |
| 2 | || | 2 |
| 3 | | | 1 |
| 4 | || | 2 |
| 5 | ||| | 3 |
What this tells us:
Top tip: Always check that your frequencies add up to the total number of data items. In this example: 1 + 2 + 1 + 2 + 3 = 9 ✓
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