56 terms

bar graph

a graphical representation of categorical data. Names of each category are listed on the x axis and a bar

is place over each category name having height equal to the frequency (or percentage) in that category.

is place over each category name having height equal to the frequency (or percentage) in that category.

bias

A condition that occurs when the design of a study systematically favors certain outcomes.

blocking

The grouping of individuals according to some characteristic like rats in the same litter or plots of land

at the same location. The random allocation is carried out separately within each group.

at the same location. The random allocation is carried out separately within each group.

boxplot

A plot of data based on the five number summary. A line is drawn from the minimum observation to Q1;

a box is drawn from Q1 to Q3 with a vertical line at the median and a line is drawn from Q3 to the maximum

observation.

a box is drawn from Q1 to Q3 with a vertical line at the median and a line is drawn from Q3 to the maximum

observation.

categorical variable

A variable that can be classified into groups or categories such as gender, religion, zip-code,

etc. Typically, words are used to describe an individual.

etc. Typically, words are used to describe an individual.

comparative study

A study where the explanatory variable has two active treatments rather than an active

treatment versus a control. Purpose of the study is to determine which treatment works best rather than whether

a treatment works. Randomization together with comparison enables the researcher to control lurking variables

and apply the laws of probability for inference.

treatment versus a control. Purpose of the study is to determine which treatment works best rather than whether

a treatment works. Randomization together with comparison enables the researcher to control lurking variables

and apply the laws of probability for inference.

completely randomized design

An experimental design where all individuals participating in the experiment are

assigned at random to the treatments.

assigned at random to the treatments.

confounded variable

A variable whose effect on the response variable cannot be separated from the effect of the

explanatory variable on the response variable. (Note: Usually confounded variables are lurking variables but

only a few lurking variables are also confounded.)

explanatory variable on the response variable. (Note: Usually confounded variables are lurking variables but

only a few lurking variables are also confounded.)

confounding

A situation where the effect of one variable on the response variable cannot be separated from the

effect of another variable on the response variable.

effect of another variable on the response variable.

control

An 'inactive' treatment where either no experimental condition or a placebo is applied to the individuals

in order to determine whether the active treatment works. Randomizing together with a control enables the

researcher to manage lurking variables when there is not a comparison group. Note: a control is not necessary

for a valid experiment as long as two or more comparison treatments are used.

in order to determine whether the active treatment works. Randomizing together with a control enables the

researcher to manage lurking variables when there is not a comparison group. Note: a control is not necessary

for a valid experiment as long as two or more comparison treatments are used.

convenience sample

A sample where the researcher contacts those subjects who are readily available and does

not use any random selection. The results are almost surely biased.

not use any random selection. The results are almost surely biased.

distribution

A table or a graph that give the possible values of a variable together with the frequency of each value.

dotplot

A one dimensional plot of a quantitative data set where each value in the data set is represented by a dot

above its corresponding location on the x axis.

above its corresponding location on the x axis.

double blind

neither the subject nor the diagnostician (doctor, nurse or whomever is diagnosing the results) knows

which treatment the subject received. Note: the researcher has to know in order to analyze the results.

which treatment the subject received. Note: the researcher has to know in order to analyze the results.

experiment

A study where a treatment is deliberately imposed on each individual in the study before responses

are measured in order to observe responses to the treatment. A valid experiment must have 1) control or

comparison, 2) randomization and 3) replication.

are measured in order to observe responses to the treatment. A valid experiment must have 1) control or

comparison, 2) randomization and 3) replication.

explanatory variable

A variable that may or may not explain the outcomes (responses) of a study. It is given as a

phrase that describes all possible treatments. Note: An observational study can have an explanatory variable,

but a valid experiment always has an explanatory variable.

phrase that describes all possible treatments. Note: An observational study can have an explanatory variable,

but a valid experiment always has an explanatory variable.

factor

Another term for explanatory variable.

first rule of data analysis

plot the data.

five number summary

minimum, Q1, median, Q3, maximum; preferred when data are very skewed or have

outliers.

outliers.

histogram

A graphical display of a quantitative data set; data are separated into intervals of equal width and a bar

is drawn over the interval having height equal to the frequency (or percentage) of values in the interval. Values

of the variable are given on the x axis and frequencies (or percentages) are given on the y axis. (Hence, a

histogram gives a distribution.) Histograms are described by shape, center and spread.

is drawn over the interval having height equal to the frequency (or percentage) of values in the interval. Values

of the variable are given on the x axis and frequencies (or percentages) are given on the y axis. (Hence, a

histogram gives a distribution.) Histograms are described by shape, center and spread.

individual

the basic unit (or subject) of the experiment upon which a treatment is applied

interquartile range (IQR)

a measure of variability recommended for skewed data or data with outliers; computed

as IQR = Q3 - Q1

as IQR = Q3 - Q1

lack of realism

A weakness in experiments where the setting of the experiment does not realistically duplicate the

conditions we really want to study.

conditions we really want to study.

left skewed

A density curve where the left side of the distribution extends in a long tail. (Mean < median.)

lurking variable

A variable that has an important effect on the relationship among the variables in a study but is

not taken into account. (Technically, a true lurking variable "interacts" with the explanatory variable in its

effect on the response variable.)

not taken into account. (Technically, a true lurking variable "interacts" with the explanatory variable in its

effect on the response variable.)

mean

A measure for the center of the data.; it's the point that "balances" the data.

median

A measure of the center of data; it's the point such that half the numbers are smaller and the other half are

larger (the midpoint of the ordered data set).

larger (the midpoint of the ordered data set).

multi-stage sample

sampling is conducted in stages; for a two-stage sample, the individuals are grouped

according to some characteristic—groups are first randomly selected and then individuals are randomly selected

from only those selected groups. (In a stratified sample, individuals are randomly selected from every group.)

For example, states could be randomly selected; then school districts within selected states, followed by schools

within selected school districts within selected states and finally students would be randomly selected from the

selected schools from the selected school districts from selected states. That would be a four-stage sample.

according to some characteristic—groups are first randomly selected and then individuals are randomly selected

from only those selected groups. (In a stratified sample, individuals are randomly selected from every group.)

For example, states could be randomly selected; then school districts within selected states, followed by schools

within selected school districts within selected states and finally students would be randomly selected from the

selected schools from the selected school districts from selected states. That would be a four-stage sample.

non-response bias

bias resulting when individuals selected to be in a survey either cannot be contacted or refuse

to answer survey questions.

to answer survey questions.

Normal distribution

A bell-shaped symmetric density curve used to model many data sets that have a symmetric

mound or bell shape.

mound or bell shape.

observational study

A study that merely observes conditions of individuals in a population and records

information; the population is disturbed as little as possible. (Note: treatments are not imposed on units.)

information; the population is disturbed as little as possible. (Note: treatments are not imposed on units.)

outlier

An observation that falls outside the overall pattern of the data set. An observation is declared an outlier if

observation < Q1 - 1.5 IQR or if observation > Q3 + 1.5 IQR.

observation < Q1 - 1.5 IQR or if observation > Q3 + 1.5 IQR.

pie chart

A graphical display of categorical data using a "pie"; each category is represented as a slice where the

size of the slice is proportional to the percentage of data in that category. Not recommended by statisticians.

size of the slice is proportional to the percentage of data in that category. Not recommended by statisticians.

placebo effect

The response of patients to any treatment even though it has no physical effect.

population

The entire group of individuals about whom we desire information.

Q1

A location measure of the data such that has one fourth or 25% of the data is smaller than it. Found by dividing

the ordered data set in half (excluding the middle observation if n is odd) and finding the median of the lower

half of the data

the ordered data set in half (excluding the middle observation if n is odd) and finding the median of the lower

half of the data

Q3

A location measure of the data that has three-fourths or 75% of the data is smaller than it. Found by dividing

the ordered data set in half (excluding the middle observation if n is odd) and finding the median of the upper

half of the data.

the ordered data set in half (excluding the middle observation if n is odd) and finding the median of the upper

half of the data.

quantitative variable

A variable with numerical observations that have value such as height or weight.

random number table

A table of digits consisting of digits 0 through 9 whose order cannot be determined but in

the long run, each digit occurs 10% of the time.

the long run, each digit occurs 10% of the time.

randomization

A method of assigning individuals in an experiment to treatment groups using some random

device that eliminates bias and gives each unit the same probability of being assigned to any treatment group.

Randomization "balances" the treatment groups, thus averaging out lurking and extraneous variables. Allows us

to use the laws of probability to make inferences.

device that eliminates bias and gives each unit the same probability of being assigned to any treatment group.

Randomization "balances" the treatment groups, thus averaging out lurking and extraneous variables. Allows us

to use the laws of probability to make inferences.

range

The maximum observation minus the minimum observation. Given as one number in statistics (i.e. If max

= 98 and min = 12, then range = 98 - 12 = 86)

= 98 and min = 12, then range = 98 - 12 = 86)

replication

Having more than one individual in each treatment group. Replication is necessary for measuring

variability. Also, the greater the replication, the more precise the results.

variability. Also, the greater the replication, the more precise the results.

response bias

bias resulting from individuals in a sample lying or giving incorrect response because they do not

have knowledge about the question or can't recall; response bias could also result from wording of the question

or from interviewers influence the responses either intentionally or unintentionally.

have knowledge about the question or can't recall; response bias could also result from wording of the question

or from interviewers influence the responses either intentionally or unintentionally.

response variable

A variable that gives the result (may not be a number) of the outcome of a study. Describes the

measure on each individual.

measure on each individual.

right skewed distribution

A density curve where the right side of the distribution extends in a long tail; (mean >

median)

median)

sample

a subset of individuals in the population; the group of individuals about which we actually collect

information.

information.

simple random sample:

A sample of size n selected from the population in such a way that each possible sample

of size n has an equal chance of being selected.

of size n has an equal chance of being selected.

standard deviation

A measure of the "average" or typical deviation of the observations about the mean;

measures variability of data about the mean

measures variability of data about the mean

standard Normal curve

A normal distribution with mean of zero and standard deviation of one. Probabilities are

given in Table A for values of the standard Normal variable.

given in Table A for values of the standard Normal variable.

statistically significant

Results of a study that differ too much from what we expected because of randomization

to attribute to chance.

to attribute to chance.

stemplot

A graphical representation of a quantitative data set. Leading values of each data point are presented as

stems and second digits are given as leaves

stems and second digits are given as leaves

stratified sampling

A sampling scheme where the population has been divided into strata (groups) according to

some characteristic and a random sample is selected from within each stratum (group).

some characteristic and a random sample is selected from within each stratum (group).

symmetric distribution

a density curve where the right half is a mirror image of the left half of the distribution.

(Mean = median)

(Mean = median)

undercoverage bias

Bias that occurs because the list of the population from which the sample is drawn is

incomplete—meaning that some people in the population are not listed for selection.

incomplete—meaning that some people in the population are not listed for selection.

voluntary response sample

A method of sample selection that consists of people choosing themselves by

responding to a general appeal.

responding to a general appeal.

z-score

A measure of the number of standard deviations a value or observation is from the mean, a standardized

value.

value.