Ch 09 Solutions

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Chapter 9

Sampling design

9.1 Learning activity A9.1

Question:

Think of at least three lists you could use in your country as a basisfor sampling. Remember, each list must:

be generally availablebe up-to-date

provide a reasonable target group for the people you might wishto sample.

Solution:

Everyone will be from different countries and I fear this subjectguide writer cannot make a compendium of all possible lists!However, it is a good idea to think about the following:

Generally available lists include registers of electors, addresscodes used by the post ofce, lists of schools, or companies.

Whether they are up-to-date or not depends on the purposefor which the list was intended. For example, looking at thekinds of lists mentioned above you would expect there to besome government regulation about registering electors (Every year? Before each national election? What is the policy in yourcountry?)

For the register of electors, the post ofce will register newbuildings by postcode on a continuous basis.

For address codes you need to check who would beresponsible for the lists of schools or companies and why and when they would be updated. Is there a subscription topay annually, for example, in order to be a member of aschool or company association, or is there a governmentregulation which means that schools or companies must beregistered?

The question of target group also affects the usefulness of your frame. If you want to know about foreign nationalsresident in your country, then the register of electors is hardly appropriate, nor will it help you if you are interested primarily in contacting people under voting age.

The list of new buildings will help you get in touch with peopleliving in particular areas so you may nd you will stratify by thesocio-economic characteristics of the place rather than the people.

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Statistics 1 Solutions to learning activities

Schools and companies are ne as a contact for people for many purposes but the fact that they have been approached through aparticular organisation may affect responses to particular questions.Would you tell interviewers who contacted you in school time andhad got hold of you through your school that you hate school, oreven about what you do after school? Would you reply to apostal/mail questionnaire sent via your company if you were askedhow often you took unnecessary sick-leave?

The more you think about all this, the more difcult it can seem! Sothink of three lists and then list the ways you could nd them useful asa basis for sampling and the problems you might have in particularcircumstances.


Question:

Think of three quota controls you might use to make a quota sampleof shoppers in order to ask them about the average amount of money they spend on shopping a week. Two will be easy for yourinterviewer to identify. How can you help them with the third?

Solution:

The two quota controls which it is relatively easy for yourinterviewer to identify are age and gender .

They are also useful controls to use as they help you gain arepresentative picture of shopping patterns. We would expectpeople to buy different things according to whether they are women(particular perfumes, for example) or men (special car accessories)or by age (out-of-season holidays for older people, or pop concerttickets for the younger) to give trivial examples. And if were wrong,and older people are into pop concerts of lots of women want tomake their cars more sporty, then we nd this out if we have madesure we have sufcient numbers in each of the controls.

The question of a third control is more tricky. We would like toknow about peoples preferences if they spend more or less money when they shop, or according to their income group, or dependingon how many people they are shopping for (just themselves, or theirfamily and their elderly neighbours).

Of course, the interviewer could ask people how much they spent onshopping last week, or what their family income is, or how many people they are shopping for. People might reply to the last of thesequestions, but may well be unhappy about the other two so youget a lot of refusals and lose your sample!

Even if they do reply, the interviewer will then have to discard someof the interviews she has started. If everyone she has interviewed sofar is shopping for themselves, for example, and she has lled herquota, she will have to ignore the person she has stopped and go to

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CHAPTER 9. SAMPLING DESIGN

look for someone else to ask whether they have shopped for others!

If the aim is to interview people who have bought a lot of things atthat time, then the interviewer could wait for people to leave thestore concerned, and stop people who have bought a lot that day, amedium amount, or a small amount, judging by how full theirshopping trolley is! Or do the same if people are accompanied by children or not on their shopping expedition.

An alternative is to interview at more than one shopping area anddeliberately go to some shops in rich areas and some in poor areas,knowing that most of the people you interview will be in an incomecategory that matches their surroundings.


Question:

Find out about one of the Government surveys carried out in yourown country and write down details regarding survey frequency,sampling frame, type of respondent, location, effective sample sizeand response rate.

This should help you understand the problems involved in designinga useful survey and help you with illustrations for your examinationquestions. (Remember that your understanding of the generalpoints raised here should be illustrated by examples. The examiners

are very happy if you give examples from your own country or areaof interest. They are not looking for points memorised fromtextbooks.)

Solution:

Im afraid I really cant help you here! You must do this yourself. If itis difcult to nd out about surveys in your home country, then look very carefully at Social Trends (given in your reading list) and readthe descriptions of, for example, the General Household Survey (GHS), the Tourism surveys, and the School leavers surveys. They

should tell you details of:

Sample frame

Respondents

Coverage of the questionnaires

Response rates

General subjects covered

You could also get hold of one of these through your library (theGHS is a good one) and see the careful explanation of how thesurvey works and the things it is used for.

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e) a random sample of the general population about whether they liked yesterdays television programmes.

Solution:

There are no absolute right answers to these you may disagree with the subject guide suggestions but if you do make sure you canexplain why you think what you do! In addition, the explanationmust use the kinds of arguments given in this chapter.

a) This slightly depends on the age of the children concerned. The younger the child, the more difcult it will be to elicit a clear writtenreply. Telephone is probably not an option (annoying to parents).On the whole, interviews at school make most sense though wehope the childrens replies wont be too inuenced by theirenvironment.

b) The items are non-essential we need clear records probably a diary kept at home (if people will agree to do this) would be best.Random digit dialling might work but you would need to catchpeople while they still remembered their shopping. Interviewingoutside the store is not an option we have been told this is arandom survey so we will need a list of addresses rather than aquota approach to people as they shop.

c) A quota sample is specied, so a face-to-face interviewer contactmethod would be appropriate. Suitable quota controls need to beprovided to the interviewer to carry out the exercise in, say, ashopping centre.

d) I wonder if email might be a good idea quick and easy. But of course, if their computer facilities are really poor, we will only getreplies from the employees with state-of-the-art facilities! If thecompany is supporting this, you could use any method aninterview might obtain detailed and useful information. If we only want to know the proportions of employees who have experiencedparticular problems with their computers, a mail survey would work and the response rate should be as good as they are doing it fortheir employer!

e) Here time is important mail will be too slow. Even a randomface-to-face survey might take too long you need to catch theinterviewee at home.

So the choice is:

If random, use randomised digital dialling and phone people athome the day following the TV programmes everyone willremember them and you can make your questions short and tothe point!

Or you may prefer to use a quota, interview, survey with thepossibility of showing pictures to illustrate your questions andperhaps ask more detailed questions. You can also do this

straight after the TV programmes go out.

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CHAPTER 9. SAMPLING DESIGN


Question:

a) Outline the main stages of a random survey. Where do the maindangers of errors lie?

b) Why might you carry out a quota survey rather than a randomsurvey?

c) The designing of questionnaires and the training of interviewersis a waste of money. Discuss.

d) When would you carry out a telephone survey rather than using aface-to-face interview?

e) You have been asked to survey the interest of a population in anew type of audiotape. How might you stratify your sample?Explain.

Solution:

a) The main points you should make are:

Deciding on your target group. Be careful is there somethingparticular about them (i.e. children who will nd writingdifcult, employees who might not feel comfortable to beinterviewed about work at their workplace) which may affect

your next stage?The sample frame it should be complete (and appropriate tothe target see above), up-to-date, not have duplicates, becentrally and cheaply available.

Sample design what are your stratication and clusteringfactors you want to maximise accuracy for cost with respectto the variables in which you are interested.

Contact method will you interview face-to-face, usetelephone, mail survey, or email, or what? This may be limitedby your sample frame. You cant ring people up, for example, if you only have their addresses.

Analysis everything youve learned in the other chapters!Errors can happen at each stage your frame may notadequately reect your target, the clusters in your design may cause more variability than you expected, people may refuse toanswer, not be at home for interviews, lose your mailquestionnaire, misunderstand your questions or lie, youranalysis may be faulty.

b) You might carry out the quota because:

You have no convenient sample frame.

You are in a hurry.

You are not too concerned at this point about measuringaccuracy you just want to get a rough idea about the problem you are studying.

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Money is a little tight and a random sample would cost toomuch.

c) Hint you should look at all stages of a survey. How many of them involve interviewers making complicated surveys? Howimportant is that? Is it a waste of money? But the kind of information think about it you can get is really impossible toget without a trained interviewer. Go into this argument in depthand think of examples.

d) The following circumstances are worth thinking about:

Am I in a hurry? Do I need a random sample? (i.e. quota wontdo on this occasion)

Will I be able to ask questions without needing to use visualaids?

e) You need to think about what might affect interest in this newaudio-tape so you can segment the possible market. Theinformation you have may be limited.

You could look at the gures for the amount of money spent onelectrical goods by sales area, and stratify by area high,medium and low spend. This would mean that you would belikely to be able to contact people with different degrees of interest in your product. This would form a good basis for arandom design.

If you are doing this study for a company that had its customersales gures, you could stratify by high, medium and low spendon appropriate items and sample from your customer base.

If you are intending to make a quota sample, you could stillstratify the areas or shops where you send your interviewers by using information about the area or customer database (if youhave it) and taking interview points which represent high,medium and low likely expenditure on appropriate products.

Solutions prepared by Dr James Abdey.

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Ch 09 Solutions