What is an estimate?
What is an estimate?
Estimating means roughly calculating or judging a number or value.
Children begin estimating in Reception: they might be given a group of objects and asked to guess how many there are. The idea is that they use their existing knowledge to make an educated assumption (often called a 'clever guess'). They would then be asked to count the objects in order to check how close they were.
Using estimation to help with calculations
Throughout their time at primary school, children are expected to be able to make estimates in order to check whether their answers are correct.
This is often done by rounding, for example: in Key Stage 1, a child might be asked to add 12 and 13. A way to estimate the answer would be to remember that both numbers are close to 10, and 10 plus 10 is 20, so if their answer were 35, they would know that this question would need re-doing.
As children move into Key Stage 2, they will use rounding, knowledge of number facts and the inverse operation to estimate answers.
For example: a child in Year 3 might be asked to work out 318 + 298.
- Imagine they got the answer 916.
- To check if this answer is correct, it would be a good idea for them to round both the numbers to the nearest hundred and then work out 300 + 300 in their heads.
- They would see that the correct answer should be somewhere around 600 and so their original answer of 916 must be incorrect.
A child in Year 4 might be asked to work out 1490 - 818.
- Imagine they got the answer 225.
- To check if this answer is correct, they could round both numbers to the nearest 100 which would be 1500 - 800.
- They could then do this calculation in their heads to get 700.
- This answer is very far off from 200, so they would know that their first answer was wrong.
In Year 5 and 6, children would be expected to use estimation when multiplying and dividing.
- For example: if they had multiplied the numbers 29 x 51 and got 720, to check this answer, they could round the numbers to their nearest tens and then mentally work out 30 x 50.
- The answer to this is 1500, which makes it clear that the original answer was very far off.
Estimation is very useful when dividing, especially if using chunking, as chunking involves working out roughly how many times one number will go into another using existing knowledge about other, smaller numbers.
A child may be asked to work out the following: 207 ÷ 23 =
- Here it would be useful for them to remember the fact that there are four 25s in 100. Therefore 23 will also go into 100 four times.
- The big number we are dealing with is close to 200, so we know that 23 will go into 207 roughly eight times.
- If we work out 23 x 8, we get 184, which is not big enough, so we can then try 23 x 9, which gives us the answer of 207!
A child may also be asked to work out the following: 405 ÷ 9 =
- They could use their knowledge of the 9 times table: 9 x 4 = 36.
- Therefore 9 x 40 = 360.
- They could then estimate how much higher the 40 would need to go in order to get the answer, for example: they could try 9 x 42 which would give the answer 378.
- They could then try 9 x 45 which would get us our answer of 405.
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