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Populations, Samples and Censuses

A population is the set of individuals in a group. Ofteninformation needs to be collected about the population, Taking acensus and taking a sample are methods of collecting data about thepopulation.

A govenrnent for example needs to plan the supply of publicservices in advance. A hospital might cost £100m to build, andshould not be built if the demand for it is not there. For thisreason, the local population needs to be accurately forecast, alongwith it age distributionand other relevant quantities.

Every ten years the governments in many countries conductcensuses. Every individual completes a form to submit all relevantinformation and the government uses to to guide government policiesand priorities. Conducting a census gathers complete information butis an expensive and time consuming process. A form must be deliveredto every house, completed and returned. If it is not returned, oftena representative must call on that house in person to chase up theform. The patience of government for this is limited. Many people donot compete a census form full stop – they are out of the countryduring the time of the census, they cannot read, they cannot bebothered, they don't believe in government anyway...- and there comesa point when the government stops chasing them for a response.Alternatively the entire census is conducted by personal interview,with government representatives knocking on doors until someoneanswers (or they give up).

A sample is easier, simpler, quicker and cheaper to conduct. Onlya representative fraction of the population need be sampled, withmen, women, old, young etc being sampled in the same proportion asthey occur in the population.

A sample is much less accurate than a census and has a margin oferror, often quoted. For example, political opinion polls are quotedasBeingmuch cheaper, many more of these can be conducted, and if the subjectof the poll is voting intentions for example, trends may be trackedover time, with several such being conducted each month, often byprivate organisations – newspapers, politicals parties andbroadcasters. These are published, agglomerated and then muchanalysed, especially in the period before elections.