01_data_collection
In scientific studies, data collection can fall under two broad categories

observational study  where data is recorded without interfering with the process under study. These can be of 3 basic types
 sample survey  provides info about a population at a particular point in time
 prospective study  describes the population at present using sample survey and proceeds to follow forward in time to record specific outcomes
 retrospective study  describes the population at present using sample survey and records specific occurrence that have already taken place
 experimental study  where explanatory variables are actively manipulated and the effects on response variables are studied.
Sampling designs for surveys  observational study¶ ¶
The various ways in which data collection experiments can be designed:

Simple random sampling  Selecting a group of n units in such a way that each sample of size n has the equal probability of being selected.

Stratified random sampling  grouping the target population into strata based on a known auxiliary variable and then performing simple random sampling on each of the strata

cluster sampling  this involves selecting groups (based on convenience of study, such as buildings or city blocks) based on simple random sampling and then surveying or measuring all units within the groups.

systematic sampling  this involves selecting every nth value from a finite list as the sample. This technique is economical but can he heavily biased.
Design of experiments  experimental study¶ ¶
There are 2 types of variables in an experimental study  controlled variables (factors) and measured variables (response variables) which are under study

factorial treatment  each of the various factors (m,n,o..) affecting the phenomena are combined with oneanother. The resulting combinations (mno..) are studied individually

fractional factorial treatment  as the number of factors increase, factorial treatment becomes impossible to complete. Then, only a few selected combinations are picked for the study forming a fractional factorial treatment.