## 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.

lum chan says

Your video really helped me understand the stratified sampling

Wolves Ay We says

do you have a video about arithmetic and geometric sequences?

Ava Flower says

Ok I watched your video 3 times and I think I get it now. Thank you Sir 🙂

avais khan says

very helpful thanks

AnnMarie Perez says

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?

Zay Soyf says

its a shame i didnt see this before yesterdays exam.

it came up on AQA paper and i didnt have a clue

Scott Clare says

coolest cardigan on youtube

lala loopsy says

And here’s another comment out of appreciation

lala loopsy says

Thanks it helped me a LOT

ashita chopra says

Wow☺thanks, totally helped …!

ashley matthews says

What if it’s a percentage sample e.g a 10% sample, stratified by time, is

taken

Evelyn Lauron says

I really learn from your presentation. Thank you for refreshing my

knowledge on Stratified sampling

Evelyn Lauron says

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.

Marvin Sean says

Thanks Simon your video really helped

mufcbenm says

thank u very much. was a huge help

Georgina Frimpong says

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!

Carmella Loyola says

Big help in making my methodology 🙂

TZEEBOY maseko says

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

Yatin Singh says

Thanks a lot, as it is very helpful.

alex l says

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?

Ryan Varghese says

Thanks …. been stressing over this topic … I now understand it ….

thanks for the great help.

Hallam Brown says

Thank you very much! You really helped me finish my math assignment 🙂

Julia kayondo says

thanks was strugling but got it

Ripu Agnihotri says

thanx great help regards

Android Man says

thanks you are you so helpfulll

James Rock says

Thanks so much!

Asif majid says

THANKS SIMON REALLY APPRECIATE YOUR VIDEOS THEY HELP A LOT

THANKS AGAIN.

The Original Dragonite says

Thanks Simon 😀

Abdala Hijazin says

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 ? :))

Bryan Kyouran says

Doing Bsc Economics we have to understand the basics. This video helped

with that, Thanks 😉

Victoria Waters says

Thank you for your help, you explained this much better than my teacher

Mojo Rapper Music says

I’v got my GCSE maths next week dreading it you saved my life!

Thanks a million

Mohamad Khalil says

Thanks a lot

sergio adamov says

Thanks…great job… Keep doing what ur doing…I’ll get my friends to

check u out… 🙂

Lisa Whitear says

Hi I am a mature student taking my maths gcse soon and find your video’s

really helpful…thanks.

Lisa Whitear says

Hi I am a mature student taking my maths gcse soon and find your video’s

really helpful…thanks.

Zomfire says

thats the exact question i have for homework, you just made it soo much

easier 😀

David Smart says

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?

V Tan says

Thank you 🙂 This video really helped me with understanding sampling in

preparation for the A levels!

Nathan Martin says

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!

sas kr says

thank you, after this video, i came to know that sampling is not an idea of

picking and problems can also come like this

Anj M says

Thanks very much for creating this video, it was very beneficial 🙂

Rima K says

Thank u!! This helped alot

Jayan Liyanage says

Thanks Simon 🙂

Simon Deacon says

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

Jayan Liyanage says

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

Kendal Luna says

oh sorry I didn’t reply and yes it did help thank you very much!

Simon Deacon says

Hi Tony – there’s different types of sampling but, usually with GCSE, these are the types of questions.

Tony Grieg says

Is this the only method we have for startified sample?

Sammi Maslen says

Most helpful maths video I’ve ever watched, thanks very much 🙂

Simon Deacon says

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!

Kendal Luna says

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

Simon Deacon says

That’s great, am pleased it helped – 🙂

Zeelan Saeed says

Thank you very much 🙂 It helped me a lot.

lordmoonaz69 says

Ok, thanks 🙂 your doing vids about what im learning at the minute, so aids revision 🙂

Simon Deacon says

Hi – no, sorry, although Mrs D works with English students at First Class Learning

lordmoonaz69 says

Theres a mrs deacon who teaches at my school, any relation?