Statistics – Chapter 1 Data Collection 1.2 Observational Studies Objectives: - Distinguish between an observational study and an experiment -Obtain a simple random sample

Sources of Data Data can be obtained from four sources: 1. Census: U.S. Census: Every 10 years, costs $6 billion, tells how many people live in household, income, etc., 100% certainty, EX POST FACTO List of all individuals in a population along with certain characteristics about each

Sources of Data 2. Existing sources: A researcher would rather use data that already exists than recollecting all that data. 3. Survey sampling: used when there is no attempt to influence the value of the variable of interest. Data obtained from survey sampling leads to an observational study

Observational Study: 4. Designed Experiments Measures characteristics of a population by studying individuals in a sample, but does not attempt to influence the variable of interest Applies treatment to individuals and attempts to isolate the effects of the treatment on a response variable

Observational Study vs. Designed Experiment Observational Study is after the fact (_________________) Observational Study does not allow the researcher to claim causation, only association! Observational Studies are used to ______________________________________ and ____________________________________ Ex Post Facto Learn characteristics of populations Determine if there is an association between 2 or more variables

Smoking vs. Lung Cancer Let’s say we were to observe characteristics of people who are smokers, and we want to see how many end up with lung cancer. What would we have to do? Would we use a designed experiment or an observational study?

Observational Study vs. Designed Experiment If control is possible… If control is not possible (because sometimes you just can’t or it’s not ethical)… Perform a Designed experiment Observational Study

Sampling Techniques Simple Random Sample Stratified Sample Goal: To obtain individuals for a study in such a way that accurate information can be obtained. The 4 Basic Sampling Techniques are: Simple Random Sample Stratified Sample Systematic Sample Cluster Sample

Simple Random Sample Simple Random Sample The most basic sample survey design Also known as random sampling A sample size n from a population of size N is obtained through if every possible sample of size n has an equally likely chance of occurring Ex. Picking out of a hat, lotto ping pong balls, Random number generators Simple Random Sample

We don’t want to survey the same person twice! Simple Random Sample Random numbers are assigned to each individual in the population (from 1-N), then n random numbers are selected from the list A frame is a list of all individuals in the population (A legend if you will) Simple random sampling always needs a frame Sampling without replacement is used if We don’t want to survey the same person twice!

How do I obtain a SRS? You need to start with a frame Arrange the individuals of the frame in some manner (i.e. alphabetically) Number the individuals from 1 to N Use some technique to pick n random numbers Random number table Random number generator in calculator

How do I obtain a SRS? Be sure to note if we are sampling with or without replacement The individuals corresponding to the n random numbers are picked to serve as the sample.

How to use RNG on TI-84 1) Set the Seed 2) randInt(a,b,n) This provides an initial starting point for the calculator. Any non-zero number # STO-> rand 2) randInt(a,b,n) a = min value of frame b = max value of frame n = number of values desired