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Why and how to use stratified sampling

February 26, 2013 by Simon 57 Comments

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.
Twice as many boys as girls

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:

Stratified sample question

  1. The government decides to use a 10% stratified sample. Write down the numbers from each category they should sample. (3 marks)*
  2. Give one other factor they should take into account when selecting, to ensure an unbiased sample. (1 mark).
Working with stratified sampling isn’t limited to samples from a population of people. It is used in many different applications such as:
  • 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.

 

 

Comments

  1. lum chan says

    August 2, 2015 at 1:14 am

    Your video really helped me understand the stratified sampling

    Reply
  2. Wolves Ay We says

    June 12, 2015 at 1:27 pm

    do you have a video about arithmetic and geometric sequences?

    Reply
  3. Ava Flower says

    June 7, 2015 at 8:55 pm

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

    Reply
  4. avais khan says

    June 6, 2015 at 7:11 pm

    very helpful thanks

    Reply
  5. AnnMarie Perez says

    June 6, 2015 at 4:06 pm

    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?

    Reply
  6. Zay Soyf says

    June 5, 2015 at 7:16 pm

    its a shame i didnt see this before yesterdays exam.
    it came up on AQA paper and i didnt have a clue

    Reply
  7. Scott Clare says

    May 29, 2015 at 11:25 am

    coolest cardigan on youtube

    Reply
  8. lala loopsy says

    April 24, 2015 at 6:33 pm

    And here’s another comment out of appreciation

    Reply
  9. lala loopsy says

    April 24, 2015 at 6:32 pm

    Thanks it helped me a LOT

    Reply
  10. ashita chopra says

    April 23, 2015 at 3:48 pm

    Wow☺thanks, totally helped …!

    Reply
  11. ashley matthews says

    April 19, 2015 at 3:48 pm

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

    Reply
  12. Evelyn Lauron says

    April 13, 2015 at 5:00 pm

    I really learn from your presentation. Thank you for refreshing my
    knowledge on Stratified sampling

    Reply
  13. Evelyn Lauron says

    April 13, 2015 at 4:55 pm

    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.

    Reply
  14. Marvin Sean says

    April 8, 2015 at 7:33 pm

    Thanks Simon your video really helped

    Reply
  15. mufcbenm says

    March 26, 2015 at 8:24 pm

    thank u very much. was a huge help

    Reply
  16. Georgina Frimpong says

    March 23, 2015 at 8:00 pm

    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!

    Reply
  17. Carmella Loyola says

    March 22, 2015 at 7:10 am

    Big help in making my methodology 🙂

    Reply
  18. TZEEBOY maseko says

    March 19, 2015 at 5:27 pm

    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

    Reply
  19. Yatin Singh says

    March 13, 2015 at 3:52 pm

    Thanks a lot, as it is very helpful.

    Reply
  20. alex l says

    March 10, 2015 at 10:58 pm

    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?

    Reply
  21. Ryan Varghese says

    February 28, 2015 at 12:48 pm

    Thanks …. been stressing over this topic … I now understand it ….
    thanks for the great help.

    Reply
  22. Hallam Brown says

    February 23, 2015 at 12:13 pm

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

    Reply
  23. Julia kayondo says

    February 16, 2015 at 10:29 am

    thanks was strugling but got it

    Reply
  24. Ripu Agnihotri says

    January 30, 2015 at 7:49 am

    thanx great help regards

    Reply
  25. Android Man says

    December 15, 2014 at 5:27 pm

    thanks you are you so helpfulll

    Reply
  26. James Rock says

    November 19, 2014 at 10:50 am

    Thanks so much!

    Reply
  27. Asif majid says

    November 4, 2014 at 11:36 pm

    THANKS SIMON REALLY APPRECIATE YOUR VIDEOS THEY HELP A LOT
    THANKS AGAIN.

    Reply
  28. The Original Dragonite says

    September 30, 2014 at 10:34 pm

    Thanks Simon 😀

    Reply
  29. Abdala Hijazin says

    July 11, 2014 at 5:01 pm

    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 ? :))

    Reply
  30. Bryan Kyouran says

    July 7, 2014 at 2:13 pm

    Doing Bsc Economics we have to understand the basics. This video helped
    with that, Thanks 😉

    Reply
  31. Victoria Waters says

    June 16, 2014 at 8:36 pm

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

    Reply
  32. Mojo Rapper Music says

    June 15, 2014 at 9:44 am

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

    Reply
  33. Mohamad Khalil says

    May 27, 2014 at 9:10 pm

    Thanks a lot

    Reply
  34. sergio adamov says

    May 15, 2014 at 10:41 pm

    Thanks…great job… Keep doing what ur doing…I’ll get my friends to
    check u out… 🙂

    Reply
  35. Lisa Whitear says

    May 14, 2014 at 7:50 pm

    Hi I am a mature student taking my maths gcse soon and find your video’s
    really helpful…thanks.

    Reply
  36. Lisa Whitear says

    May 14, 2014 at 7:50 pm

    Hi I am a mature student taking my maths gcse soon and find your video’s
    really helpful…thanks.

    Reply
  37. Zomfire says

    May 3, 2014 at 5:17 pm

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

    Reply
  38. David Smart says

    March 23, 2014 at 10:42 am

    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?

    Reply
  39. V Tan says

    March 15, 2014 at 4:38 pm

    Thank you 🙂 This video really helped me with understanding sampling in
    preparation for the A levels!

    Reply
  40. Nathan Martin says

    March 6, 2014 at 1:25 am

    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!

    Reply
  41. sas kr says

    January 28, 2014 at 9:05 am

    thank you, after this video, i came to know that sampling is not an idea of
    picking and problems can also come like this

    Reply
  42. Anj M says

    January 4, 2014 at 9:40 pm

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

    Reply
  43. Rima K says

    November 7, 2013 at 8:33 pm

    Thank u!! This helped alot

    Reply
  44. Jayan Liyanage says

    August 28, 2013 at 8:55 pm

    Thanks Simon 🙂

    Reply
  45. Simon Deacon says

    August 28, 2013 at 12:43 am

    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

    Reply
  46. Jayan Liyanage says

    August 27, 2013 at 8:03 pm

    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

    Reply
  47. Kendal Luna says

    June 16, 2013 at 9:44 pm

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

    Reply
  48. Simon Deacon says

    June 7, 2013 at 2:49 pm

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

    Reply
  49. Tony Grieg says

    June 7, 2013 at 10:35 am

    Is this the only method we have for startified sample?

    Reply
  50. Sammi Maslen says

    May 19, 2013 at 12:47 pm

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

    Reply
  51. Simon Deacon says

    April 8, 2013 at 9:33 am

    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!

    Reply
  52. Kendal Luna says

    April 8, 2013 at 12:40 am

    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

    Reply
  53. Simon Deacon says

    February 23, 2013 at 10:28 pm

    That’s great, am pleased it helped – 🙂

    Reply
  54. Zeelan Saeed says

    February 23, 2013 at 7:51 pm

    Thank you very much 🙂 It helped me a lot.

    Reply
  55. lordmoonaz69 says

    February 22, 2013 at 11:32 pm

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

    Reply
  56. Simon Deacon says

    February 22, 2013 at 10:17 pm

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

    Reply
  57. lordmoonaz69 says

    February 22, 2013 at 10:05 pm

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

    Reply

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