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Random Sampling

  • As the name suggests, random sampling is one where the individual units from the population (samples) are selected at random.
  • The government wants to determine the impact of the rise in petrol price on the household budget of a particular locality.

A Representative of 20 Kuchha and 20 Pucca Houses

A Representative of sample

Anon-representative of sample
  • For this, a representative (random) sample of 30 households has to be taken and studied.
  • The names of all the 300 households of that area are written on pieces of paper and mixed well, then 30 names to be interviewed are selected one by one.
  • In the random sampling, every individual has an equal chance of being selected and the individuals who are selected are just like the ones who are not selected.
  • In the above example, all the 300 sampling units (also called sampling frame) of the population got an equal chance of being included in the sample of 30 units and hence the sample, such drawn, is a random sample.
  • This is also called lottery method. The same could be done using a Random Number Table also.
How to use the Random Number Tables?
  • Do you know what the Random Number Tables are? Random number tables have been generated to guarantee equal probability of selection of every individual unit (by their listed serial number in the sampling frame) in the population.
  • They are available either in a published form or can be generated by using appropriate software packages .You can start using the table from anywhere, i.e., from any page, column, row or point.
  • In the above example, you need to select a sample of 30 households out of 300 total households.
  • Here, the largest serial number is 300, a three digit number and therefore we consult three digit random numbers in sequence.
  • We will skip the random numbers greater than 300 since there is no household number greater than 300.
  • Thus, the 30 selected households are with serial numbers:
149, 219, 111, 165, 230, 007, 089, 212, 051, 244, 300, 051, 244, 155, 300, 051, 152, 156, 205, 070, 015, 157, 040, 243, 479, 116, 122, 081, 160, 162.

Exit Polls
You must have seen that when an election takes place, the television networks provide election coverage. They also try to predict the results.

This is done through exit polls, wherein a random sample of voters who exit the polling booths are asked whom they voted for. From the data of the sample of voters, the prediction is made.

You have to analyse the trend of foodgrains production in India for the last fifty years. As it is difficult to include all the years, you have to select a sample of production of ten years. Using the Random Number Tables, how will you select your sample?

Non-Random Sampling
  • There may be a situation that you have to select 10 out of 100 households in a locality.
  • You have to decide which household to select and which to reject. You may select the households conveniently situated or the households known to you or your friend.
  • In this case, you are using your judgement (bias) in selecting 10 households. This way of selecting 10 out of 100 households is not a random selection.
  • In a non-random sampling method all the units of the population do not have an equal chance of being selected and convenience or judgement of the investigator plays an important role in selection of the sample.
  • They are mainly selected on the basis of judgment, purpose, convenience or quota and are non-random samples.

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