What sample is large enough?
A great deal of statistical research has gone into this issue. As a
general rule, statisticians have found that for many population
distributions, once the sample size is at least 30, the sampling
distribution of the mean will be approximately normal. However, we
may be able to apply the central limited theorem for even smaller
sample sizes if a great deal of information is already known about the
target population.
Cluster Analysis
Cluster analysis is used for classifying objects or cases, and sometimes
variables, into relatively homogeneous groups. The groups of clusters
are suggested by the data and are not defined a priori. Researches at
Penn and Associates select variables for cluster analysis based on
hypothesis testing.
Data Collection Methods
There are several data collection methods and each have various
advantages.

Speed: Email and web page surveys are the fastest methods,
followed by telephone interviewing. Interviewing by mail is the
slowest.

Cost: Personal interviews are the most expensive followed by
telephone and then mail. Email and web page surveys are the least
expensive for large samples.

Internet: Web surveys offer fantastic advantages  cost, speed,
and detail.

Sensitive Questions: People are more likely to answer sensitive
questions when interviewed directly.
Factor Analysis
Factor analysis is a class of procedures used for reducing and
summarizing data. Each variable is expressed as a linear combination
of the underlying factors. Likewise, the factors themselves can be
expressed as linear combinations of the observed variables. The
number of factors that should be extracted can be determined a priori.
A rotation (varimax) transforms the factor matrix making it simpler
and easier to interpret.
