ap statistics unit four: collecting data

population

the entire group of individuals which we want information

sample

is the part of the population from which we actually collect information, used to draw conclusions about the entire population

sample survey

collects data from a sample to learn about the population from which the sample was selected

convenience sampling

selects individuals from population who are easy to reach

bias

likely to underestimate or overestimate the value you want to know

voluntary response sampling

allows people to choose to be in the sample by responding to a general invitation

random sampling

involves using a chance process to determine which members of a population are included in the sample

simple random sample

size n is chosen in such a way that every group of n individuals in the population has an equal chance to be selected as the sample

how to choose srs with technology

label, randomize, select

label

give each individual in the population a distance numerical label from 1 to N, where n is the sample size

randomize

use a random generator to obtain n different integers from 1 to N, where n is the sample size

select

choose the individuals that correspond to the randomly selected integers

when sampling WITHOUT replacement an individual from a population can be selected only once

when sampling WITH replacement an individual from a population can be selected more than once

table of random digits

is a long string of the digits 0,1,2,3,4,5,6,7,8,9 with these properties

strata

group of individuals in a population who share characteristics thought to be associated with the variables being measured in a study

stratified random sampling

selects a sample by choosing an srs from each stratum and combining the srss from each stratum and combining the srss intro one overall sample

cluster

group of individuals in the population that are located near each other

cluster sampling

selects sample by randomly choosing clusters and including each member of the selected clusters in the sample

systematic random sampling

selects sample from an ordered arrangement of the population by randomly selecting on of the first k individuals and choosing every kth individual thereafter

multistage sampling

used by most large-scale sample surveys, which combines two or more sampling methods

sampling errors

come from the act of choosing a sample. random sampling error and undercover age are common types

undercoverage

occurs when some members of the population are less likely to be chosen or cannot be chosen in a sample

sampling frame

list from which the sample is actually chosen

nonresponse

occurs when an individual chosen for the sample can’t be contacted or refuses to participate

response bias

occurs when there is a systematic pattern of inaccurate answers to a survey question

observation study

observes individuals and measures variable of interest but does not attempt to influence the responses

response variable

measures outcome of study

explanatory variable

may help explain or predict changes in a response variable

confounding

occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other

experiment

deliberately imposes treatments (conditions) on individuals to measure their responses

placebo

treatment that has no active ingredient but otherwise like other treatments

treatment

specific condition applied to individuals in an experiment

experimental unit

object to which a treatment is randomly assigned

subjects

when experimental units are human beings

factor

explanatory variable that is manipulated and may cause change in the response variable

levels

the different values of a factor

control group

used to provide a baseline for comparing the effect of other treatments

placebo

inactive treatment

placebo effect

describes the fact that some subjects in an experiment will respond favorably to any treatment, even an inactive treatment

double-blind experiment

neither the subjects nor those who interact with them and measure the response variable know which treatment a subject is receiving

single-blind experiment

either the subjects or the people who interact with them and measure the response variable don’t know which treatment a subject is receiving

random assignment

experimental units are assigned to treatments using a chance process

control

means keeping other variables constant for all experimental units

replication

means giving each treatment to enough experimental units so that a difference in the effects of the treatments can be distinguished from a chance variation due to the random assignment

the basic principles of experimental design

comparison, random assignment, control, replication

completely randomized design

the experimental units are assigned to the treatments completely at random

block

a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments

randomized block design

the random assignment of experimental units to treatments is carried out separately within each block

matched pairs design

common experimental design for comparing two treatments that uses blocks of size 2. in some matched pairs designs, two very similar experimental units are paired and the two treatments are randomly assigned within each pair. in other, each experimental units receives both treatments in a random order

sampling variability

refers to the fact that different samples of the same size from the same population produce different estimates. larger random samples tend to produce estimates that are closer to the true population value than smaller random samples. estimates from larger samples are more precise

statistically significant

observed results of a study are too unusual to be explained by chance alone

random selection

allows inference about the population from which the individuals were chosen

random assignment

allows inference about caus