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39 Cards in this Set

  • Front
  • Back
the objects described by a set of data.
any characteristic of an individual
categorical variable
places an individual into one of several groups or categories
quantitative variable
takes numerical values for which arithmetic operations such as adding or subtracting make sense
tells us what values it takes and how often it takes these values
rules for a histogram
1. The bars of a histogram should cover the entire range of values of a variable. When the possible values of a variable have gaps between them, extend the bases of the bars to meet halfway between the two adjacent possible values
describing overall pattern
shape, center, spread and check for outliers
how to make a stemplot
1. separate each observation into stem/leaf 2. Write the stems in a vertical column with smallest at top 3. Write each leaf in the row to the right of its stem in increasing order out 4. For double stems, 0-4 goes on the upper stem and 5-9 goes on the lower stem
average (x1+x2+x3….)/n
resistant measure
because the mean cannot resist the influence of extreme observations, it is not a resistant measure. Outliers may pull the mean toward its long tail and skew the mean. The median is resistant. Standard deviation is not resistant either.
how to find the median
1. Arrange all observations in order of size, from smallest to largest 2. If the number of observations, n, is odd, the median is the center. Find the location by counting (n+1)/2. If n is even, average the two center observations
relationship between the mean and the median graphically
in a symmetric distribution, they are close together. If the distribution is exactly symmetric, the mean and median are exactly the same. In a skewed distribution, the mean is farther out n the long tail than the median
How to measure spread:
look at the spread of the middle half of the data where there are not outliers, that is look at the quartiles
first quartile
lies one quarter of the way up the list
third quartile
larger than 75% of the observations
how to make a boxplot
take the 5 number summary. Draw horizontal lines at the 3rd quartile, the median and the first quartile. Then draw vertical lines to the maximum and minimum.
(s squared) of a set of observations is an average of the squares of the deviations of the observations from their mean
standard deviation
(s) measures spread by looking at how far the observations are from their mean
density curve rules
1. Always on or above the horizontal axis 2. Has area exactly 1 underneath it… describes the overall pattern of a distribution. The area under the curve and above any range of values is the proportion of all observations that fall in that range
median on a density curve
equal areas point with half the area under the curve to the left and the remaining to the right
mean on a density curve
the point at which the curve would balance if it was made of solid material. The mean and median of a symmetric density curve are equal
the notation for the mean of an idealized distribution
Normal distributions
normal curves that are symmetric, single-peaked, and bell-shaped.
68-95-99.7 rule
68% of the observations fall within 1 standard deviation on either side of the mean, 95% of the observations fall within 2 standard deviations on either side of the mean, 99.7% of the observations fall within 3 standard deviations on either side of the mean
standardized value
z= (x-mu)/alpha also known as Z score
z score
tells us how many standard deviations the original observation falls away from the mean and in which direction. Observations larger than the mean are positive when standardized and vice versa
the standard normal distribution
N(0,1) with mean 0 and standard deviation 1. If a variable x has any Normal distribution N(mu, alpha) with mean mu and standard deviation alpha, then the standardized variable is Z
response variable
measures an outcome of a study aka dependent variables
explanatory variable
explains or influences changes in a response variable also known as independent variables
shows the relationship between two quantitative variables measured on the same individuals. The values of one variable appear on the horizontal axis, and the values of the other variable appear on the vertical axis. Each indvidual in the data appears as the point in the plot fixed by the values of both variables for that individual. Always plot the explanatory variable, if there is one, on the horizontal axis of a scatterplot. Explanatory variable = x and response variable = y.
how to examine a scatterplot
chech for overall pattern, striking deviation, direction, strength, outlier
positive association
above average values of one tend to accompany above-average values of the other, and below-average values also tend to occur together
negative association
above average values of one tend to accompany below-average values of the other, and vice versa
strength of a stemplot
determined by how closely the points follow a clear form
r measures the direction and strength of the linear relationship between two quantitative variables ( you are trying to evaluate how closely points lie to a straight line)
how to assess correlation
1. positive r denotes positive correlation and negative r shows negative correlation. 2. Correlation is always between -1 and 1. Values of r near 0 indicate a weak linear relationship. The strength of the linear relationships increases as r moves toward 1 or -1. -1 or 1 only occur in the case of a perfect linear relationship. 3. Does not describe curved relationships 4. Not resistant
If X~N(6, 2)and a sample of n=4 is taken, then x bar is distributed
N(6,1) (standard deviation/root n)
If X~N (6,2) and a sample of n=4 is taken, (x-6)/2 is distributed
N(0,1) equal to Z