Post on 12-Jan-2016
© Nuffield Foundation 2011
Nuffield Free-Standing Mathematics Activity
Rain or shine?
Spreadsheet activity
© Nuffield Foundation 2011
Parents sometimes say that it rains more in August when children are not at school than it does in other summer months.
This activity will show you how to use a significance test to decide whether or not such hypotheses are likely to be true.
Some people think Scotland is sunnier in spring than England and Wales.
© Nuffield Foundation 2011
Monthly hours of sunshine England & Wales (2001 – 2010)Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec2001 74.5 86.7 90.2 134.6 226.2 192.7 190.0 177.8 113.8 103.6 65.8 75.42002 41.2 75.3 112.4 189.4 180.2 163.5 163.9 158.3 154.6 93.4 55.3 34.32003 70.5 95.3 165.2 191.3 189.9 209.4 172.5 207.6 166.0 130.3 67.4 52.12004 49.5 88.3 108.2 133.3 204.6 198.2 166.0 173.8 155.7 95.9 47.9 52.12005 56.4 72.0 79.5 146.9 216.6 190.8 178.4 210.6 150.6 81.1 90.8 57.02006 51.0 68.3 97.4 160.4 173.1 229.7 287.6 151.4 157.5 95.6 93.1 45.82007 60.5 70.4 153.9 216.5 167.1 149.8 179.7 198.1 149.3 109.4 71.2 51.22008 47.8 118.2 120.0 153.9 193.0 199.3 184.9 112.8 114.6 122.5 56.6 66.42009 56.9 55.6 153.2 170.3 212.7 202.0 177.1 173.5 142.8 90.2 65.7 62.22010 60.9 60.1 125.8 201.8 204.8 238.8 152.1 148.8 138.8 120.3 71.5 53.8
Think aboutWhat are the three averages that can be used to represent data?What measures of spread do you know? Which is the most appropriate average and measure of spread to use in this context?
To find the mean and standard deviation
The best estimator of the standard deviation of the population is:
The sample mean is given by the formula: nxx
You can use a calculator or a spreadsheet to work out the mean and standard deviation.
s=
22
xn
x
Try this Use the data on the EWSun worksheet to work out, for July then August, the mean and standard deviation of the monthly hours of sunshine.
The sample standard deviation is:
n - 1 =s
n
n
1
Monthly hours of sunshine in England & Wales
July August
Mean184.4 hours 174.8 hours
Standard deviation37.33 hours 31.47 hours
Think aboutCompare the results. How can you decide whether July is significantly more sunny than August?
This can be done by carrying out a significance test.
Testing the difference between sample means
is normal with mean 1 - 2
and standard deviation
Central Limit TheoremThis is also true for other underlying distributions if n1, n2 are large.
When samples of size n1 and n2 are taken at random from normal distributions with means 1, 2 and standard deviations 1, 2
2
22
1
21
nσ
nσ
Distribution of 21 XX
Calculate the test statistic:
Compare this value with the relevant critical value of z.
State the null hypothesis: H0: 1 = 2
H1: 1 2
Summary of method for testing the difference between sample means
and alternative hypothesis:
H1: 1 < 2
or H1: 1 > 2
2-tailed test
1-tailed test
(i.e. 1 – 2 = 0)
22
11
21
nσ
nσ
xxz22
Critical values of z for 2-tailed tests:
For a 1% significance test use z = ± 2.58
If the test statistic is in the critical regionreject the null hypothesis and accept the alternative.
For a 5% significance test use z = ± 1.96
95%
z0 1.96
2.5%
– 1.96
2.5%
State your conclusion clearly in terms of the real context.
Think aboutWhy is the critical value 1.96?
Critical values of z for 1-tailed tests
For a 1% significance test use z = 2.33 or – 2.33
If the test statistic is in the critical regionreject the null hypothesis and accept the alternative.
95%
z0 1.65
5%
For a 5% significance test use z = 1.65 or – 1.65
State your conclusion clearly in terms of the real context.
Test statistic
5031.47
5037.33
174.8184.422
z = 1.39
H0: 1 = 2
H1: 1 > 2 1-tailed test
is not significant
n1 = n2 = 50
Testing whether July is significantly more sunny than August
July August
Mean 184.4 hours 174.8 hoursStandard deviation 37.33 hours 31.47 hours
Using the 5% level
z0 1.65
5%
95%
1.39
Conclusion: July is not significantly sunnier than August.
22
11
21
nσ
nσ
xxz22
Try this
Write down and test other hypotheses comparing:
• the amount of sunshine in two other months of the year
• the amount of rainfall in two months
• the amount of sunshine and/or rainfall in the countries of the UK
Write a short report summarising your findings.
Rain or shineReflect on your work
• What is measured by standard deviation?
• When is it better to use n and when n – 1 ?
• Describe the steps in a hypothesis test.
• What do you do if the value of z is in the critical region?
• Can you use a hypothesis test to prove that a theory is true or false?