Stratified sampling is all about using a smaller sample to collect data
.. and then using the information to make conclusions about the whole population. It’s usually cheaper and quicker – so, if you want to find out how many students, in a high school, like football – you can just ask a few and decide the outcome.
That’s the theory… but there is a bit of a problem. Suppose you
- just ask girls whether they like football, would that be a fair sample?
- Or, just boys in year 7 – would that be fair?
The whole idea of stratified sampling is that
- the population is split into groups (age, gender etc)
- a random sample is taken from each group
- the random sample is proportional to the size of the group.
Watch the video and try the quick test.
Click here to try the Quick Test stratified sampling
This video is based on a grade 5 GCSE question:
“Andrew is going to carry out a survey of these students. He uses a sample of 50 students, stratified by year group and gender. Work out the number of Year 13 girls that should be in the sample.”
Another example that appeared as a GCSE question, although could be a real survey was:
“The government wants to survey students, studying science, about their views on becoming teachers. The University of Surrey is chosen as there are 2371 students.
The cumulative percentages of students studying each science subject is:
- The government decides to use a 10% stratified sample. Write down the numbers from each category they should sample. (3 marks)*
- Give one other factor they should take into account when selecting, to ensure an unbiased sample. (1 mark).
- geographical surveys to look at number of animals, types of plants, rocks or soil
- factory products – working out the likely number of defective items
.. and so on.
Please add a comment below if you can think of any others!
View the video on YouTube:
How to use stratified sampling
* Calculate 18% of 2371 and so on… then work out 10% of each number.
Your video really helped me understand the stratified sampling
do you have a video about arithmetic and geometric sequences?
Ok I watched your video 3 times and I think I get it now. Thank you Sir 🙂
very helpful thanks
I the answer came out as as a decimal place larger than 5(e.g 14.5) would
you round the number of people up?
its a shame i didnt see this before yesterdays exam.
it came up on AQA paper and i didnt have a clue
coolest cardigan on youtube
And here’s another comment out of appreciation
Thanks it helped me a LOT
Wow☺thanks, totally helped …!
What if it’s a percentage sample e.g a 10% sample, stratified by time, is
taken
I really learn from your presentation. Thank you for refreshing my
knowledge on Stratified sampling
I would like to ask how to do my Sampling. I have 4 population. They are
all grade 10students from 4 high schools. HS 1 has 155 girls & 220
boys,HS2 has 185 girls & 189 boys, HS3 has 280 girls and 250 boys, HS 4 has
99 boys & 165 girls. In each population I would like to get their learning
styles based on the ff: visual, auditory, kinesthetic with 10 Girls, 10
boys) respectively.
Thanks Simon your video really helped
thank u very much. was a huge help
This is really helpful i was really worried because I have a test tomorrow
and i have nothing to worry about. Thank you so much!
Big help in making my methodology 🙂
How many males and how many females would you include in a stratified
random sample of size 10 ?
There are females and males altogether
so the sample should contain females and males.
(b)
Select a stratified random sample of size 10.
Use the random number table below. Begin with females.
5 8 3 9 7 9 1 2 1 1 2 0 4 8 9 0 5 6 8 0 6 5 8 8 6 1 9 9 1 8 6 5 5 4 6 9 1 7
9 1 4 1 9 9 6 8 1 5 9 6
6 9 4 8 7 6 7 3 8 5 0 1 5 1 1 2 8 7 0 2 9 2 4 5 5 0 0 7 9 4 2 0 8 4 1 5 9 3
1 7 0 6 8 6 3 9 9 9 3 5
Thanks a lot, as it is very helpful.
Thank you Simon its very helpful, I have one question, what happens if the
total number of people draw from each strata doesn’t add up to the sample
size i.e population is 100 people, sample size is 10 people, number of
strata is 15 and the total number of sample from each strata (after
rounding) is 15 people?
Thanks …. been stressing over this topic … I now understand it ….
thanks for the great help.
Thank you very much! You really helped me finish my math assignment 🙂
thanks was strugling but got it
thanx great help regards
thanks you are you so helpfulll
Thanks so much!
THANKS SIMON REALLY APPRECIATE YOUR VIDEOS THEY HELP A LOT
THANKS AGAIN.
Thanks Simon 😀
heyy , the following is written in my Dr slide’s about stratified sampling :
” for a given sample size, reduces error compared to simple random
sampling if the groups are different from each other . ”
” Trade off between the cost of doing the stratification and smaller
sample size needed for same error ”
” probabilities of selection may be different for groups , as long as
they are known ”
” over sampling small groups improves inter group comparisons ”
i have some troubles understanding this , can u help ? :))
Doing Bsc Economics we have to understand the basics. This video helped
with that, Thanks 😉
Thank you for your help, you explained this much better than my teacher
I’v got my GCSE maths next week dreading it you saved my life!
Thanks a million
Thanks a lot
Thanks…great job… Keep doing what ur doing…I’ll get my friends to
check u out… 🙂
Hi I am a mature student taking my maths gcse soon and find your video’s
really helpful…thanks.
Hi I am a mature student taking my maths gcse soon and find your video’s
really helpful…thanks.
thats the exact question i have for homework, you just made it soo much
easier 😀
Thanks a lot.This video is very helpful.How can I get the video on ‘why
stratified sampling’
Also I would love to subscribe. Would you mind giving me tips as to how to
do that?
Thank you 🙂 This video really helped me with understanding sampling in
preparation for the A levels!
So once you find the sample size of the certain group (in this case: Girls
in year 13) do you use ‘simple random sampling’ to find out the exact
people? (e.g. pick out of a hat)
If yes, would that be considered ‘two stage sampling’?
Thanks!
thank you, after this video, i came to know that sampling is not an idea of
picking and problems can also come like this
Thanks very much for creating this video, it was very beneficial 🙂
Thank u!! This helped alot
Thanks Simon 🙂
HI Jayan – this is quite a difficult question and depends upon what you are looking for. If this is a real survey there are a number of sampling techniques that will minimise bias – there’s some good info online. If this is an exercise perhaps sampling employees and then customers separately … All best S
hi simon,
i have some issue in stratified sample technique, when i get the Total population customer (2M) + employee (8000),S=384, the sample size are distribute in disproportionate in resaerch. the employee level i can get few amout of sample than the customer side.Please give me suitable solution for me
oh sorry I didn’t reply and yes it did help thank you very much!
Hi Tony – there’s different types of sampling but, usually with GCSE, these are the types of questions.
Is this the only method we have for startified sample?
Most helpful maths video I’ve ever watched, thanks very much 🙂
Hi Kendal – Stratified sampling is basically about cost and time. The idea is that, if you want some information about what everyone thinks – you ask a sample and then use that to apply to the whole group. So, if you want to ask about a favourite TV show you talk to 3000 people and that represents the whole population. The ‘stratified’ bit is about making sure you ask young, older, male, female etc. Hope this helps!
Hello I just wanted to ask a simple question about stratified sampling..I know you might have said it in the video but i just want to know what are its uses
That’s great, am pleased it helped – 🙂
Thank you very much 🙂 It helped me a lot.
Ok, thanks 🙂 your doing vids about what im learning at the minute, so aids revision 🙂
Hi – no, sorry, although Mrs D works with English students at First Class Learning
Theres a mrs deacon who teaches at my school, any relation?