Populationentire set of things of interest

e.g., the entire piggy bank of penniese.g., the entire population of individuals in the US

Sample the part of the population about which you

actually have informatione.g., a handful of penniese.g., 100 men and women who answered an online

questionnaire about health care usage

It is usually more practical to obtain information from a sample than from the entire population.

The goal of research is to make generalizations or predictions about populations or events in general.

Much of social and behavioral research is conducted by evaluating a sample of individuals who are representative of a population of interest.

Population

Sample

Using data to say something (make an inference) with confidence, about a whole (population) based on the study of a only a few (sample).

Sampling Frame

Sampling Process

What you want to talk

about

What you actually

observe in the data

Inference

Random Selection method of choosing a sample in which each individual in the population

has an equal chance of being selected A sample “n” is selection from population ‘N” Selection process with no pattern; unpredictable Reduces the likelihood of researcher bias Researcher can calculate the probability of certain outcomes

Several different ways to conduct random samplingRandom numbers table, drawing out of a hat, coin flips, etc

Why Random Assignment is best? Samples that are assigned in a random fashion are most likely to be truly

representative of the population under consideration.

Haphazard Selection (Convenience Sampling) method of selecting a sample of individuals to study by taking whoever

is available or happens to be first on a list This method of selection can result in a sample that is not

representative of the population.

Sample selection is usually mentioned in the methods section of a research article.

Appropriate sampling methods must be evaluated.

All other things being equal, smaller samples (e.g., those with fewer than 1,000 respondents) have greater sampling error than larger samples. To better understand the notion of sampling error,

it is helpful to recall that data from a sample provide merely an estimate of the true proportion of the population that has a particular characteristic

Population Parametersmean, variance, and standard deviation of a

populationare usually unknown and can be estimated

from information obtained from a sample of the population

Sample Statisticsmean, variance, and standard deviation you

figure for the samplecalculated from known information

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Expected relative frequency of a particular outcomeoutcome

term used for discussing probability for the result of an experiment

expected relative frequencynumber of successful outcomes divided by the number

of total outcomes you would expect to get if you repeated an experiment a large number of times

long-run relative-frequency interpretation of probability understanding of probability as the proportion of a

particular outcome that you would get if the experiment were repeated many times

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Determine the number of possible successful outcomes.

Determine the number of all possible outcomes.

Divide the number of possible successful outcomes by the number of all possible outcomes.

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You have a jar that contains 100 jelly beans. 9 of the jelly beans are green.

The probability of picking a green jelly bean would be 9 (# of successful outcomes) or 9%100 (# of possible outcomes)

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Probability cannot be less than 0 or greater than 1.Something with a probability of 0 has no

chance of happening.Something with a probability of 1 has a

100% chance of happening.

Copyright © 2011 by Pearson Education, Inc. All rights reserved

p is a symbol for probability.Probability is usually written as a decimal, but

can also be written as a fraction or percentage.p < .05Threshold for significance in research

the probability is less than .05So, you are saying that there is less than a 5% chance

of the differences you see in your research results being from something other than random chance variation

Probability is discussed in the context of reporting statistical significance of study results. The p-value is the probability of the findings being by something other than chance occurrence.

Researcher will set the threshold prior to doing the research;

Threshold p-value is usually p< .05 or p< .01

The normal distribution can also be thought of as a probability distribution.The percentage of scores between two Z

scores is the same as the probability of selecting a score between those two Z scores.