Determination of sample size

Determination of sample size is a difficult task.
Particularly in social research, where we consider
a set of variables, it becomes really difficult to ascertain
the size of sample. However, it is possible to determine
the size of sample by using a representative sampling
procedure.
Based on the assumptions of probability sampling
it is possible to select a sampling procedure which
will be representative. This helps researchers to
estimate the extent to which the sample statistics
are likely to differ from population parameters. This
means, if probability sampling method is used, it
is possible to ascertain the size of the sample that
will be needed if we want to have a given degree of
certainty that their sample results do not differ
by more than a specified value from those of the
population parameters.
There are a number of sampling techniques which
decrease the likelihood of misleading generalisations
based on sample statistics. These techniques assure
the inclusion of diverse elements of the population
in the sample by controlling the proportions of the
various types of elements. Alternatively, statistical
procedures are used in the analysis of the data.
Krejcie and Morgan (1970) have given a table in
which no calculations are needed to determine the
size of the sample.

Table is reproduced here for

your reference.

N        S           N          S            N                S

10      10         220        140          1200          291

15     14          230        144         1300           297

20      19          240         148        1400         302

25      24         250          152       1500          306

30      28       260          155        1600            310

35      32        270         159         1700            313

40       36       280        162         1800             317

45      40       290        165          1900              320

50       44      300        169         2000               322

55        48     320        175            2200                327

60        52     340         181          2400              331

65         56     360         186          2600              335

70        59     380          191        2800               338

75       63       400          196      3000              341

80       66     420           201         3500             346

85       70      440          205         4000               351

90       73       460           210         4500              354

95       76       480            214       5000               357

100       80     500            217          6000               361

110      76       550           226        7000              364

120      92        600            234         8000               367

130      97        650            242           9000              368

140     103       700              248         10000            370

150    108        750              254           15000             375

160    113         800             260           20000            377

170     118       850              265          30000          380

180     123       900             269           40000          380

190    127         950           274           50000          381

200    132        1000           278           75000           382

210    136        1100           285         1000000         384

Note –  N is population size, S is sample of size

For example if you want to know the sample size

required to be representative of the opinions of 300

social work students, refer table at N=300. The sample

size representative of the social work students in

this case will be 169. The table given above is applicable

to any defined population.