Data: Facts or measurements that have been collected
Population: The whole group you want to learn about
Sample: A smaller group taken from the population
Random sample: A sample where every member of the population has an equal chance of being chosen
Bias: A systematic error that tilts results in one direction
Survey: A way of collecting data by asking people questions
Census: A survey of every member of the population
Variable: A characteristic that can take different values
Categorical data: Data sorted into groups or names
Numerical (quantitative) data: Data that are numbers you can measure or count
Discrete data: Data that can only take separate values, like counts
Continuous data: Data that can take any value in a range, like height
Mean: Sum of all values divided by how many values
Median: Middle value of ordered data
Mode: Most frequently occurring value
Range: Maximum minus minimum
Outlier: A value far away from the rest of the data
Quartile: One of three values that split ordered data into four equal parts
Q1 (first quartile): The median of the lower half of the data
Q3 (third quartile): The median of the upper half of the data
Interquartile range (IQR): Q3 − Q1
Five-number summary: Minimum, Q1, median, Q3, maximum
Outlier rule (1.5 × IQR): Values below Q1 − 1.5·IQR or above Q3 + 1.5·IQR are flagged as outliers
Percentile: The value below which a given percent of data fall
Variance: The average squared distance from the mean
Standard deviation: The square root of variance; typical distance from the mean
Z-score: How many standard deviations a value is from the mean: (x − mean)/s
Frequency: How many times a value occurs
Relative frequency: Frequency divided by the total count
Cumulative frequency: A running total of frequencies
Histogram: A bar graph for numerical data grouped into intervals
Box plot (box-and-whisker): A graph of the five-number summary
Dot plot: A graph with a dot above a number line for each data value
Stem-and-leaf plot: A table that splits each value into a stem and a leaf
Scatter plot: A graph of ordered pairs showing how two variables relate
Bar graph: A graph using separate bars to compare categories
Pie chart: A circle graph that shows parts of a whole
Line graph: A graph that connects points to show change over time
Symmetric distribution: The left and right sides mirror each other
Skewed right: A longer tail on the right; mean is usually greater than median
Skewed left: A longer tail on the left; mean is usually less than median
Normal distribution: A symmetric bell-shaped distribution
Empirical rule: About 68%, 95% and 99.7% of normal data fall within 1, 2 and 3 standard deviations of the mean
Correlation: A measure of the strength and direction of a linear relationship
Correlation coefficient (r): A number from −1 to 1 measuring linear correlation
Positive correlation: As one variable increases, so does the other
Negative correlation: As one variable increases, the other decreases
Correlation vs. causation: Two things moving together does not prove one causes the other
Line of best fit: A line drawn to match the trend in a scatter plot
Regression: A method for fitting a model to describe how one variable depends on another
Residual: Actual value minus predicted value
Extrapolation: Predicting beyond the range of the data
Interpolation: Predicting between known data points
Hypothesis test: A procedure for deciding whether data support a claim
Null hypothesis: The default claim that nothing special is going on
P-value: The probability of results at least as extreme as observed if the null hypothesis is true
Confidence interval: A range of values likely to contain the true population value
Margin of error: The amount added and subtracted from an estimate to form an interval
Sampling distribution: The distribution of a statistic over many samples
Central limit theorem: Sample means tend toward a normal shape as sample size grows
Control group: The group in an experiment that does not get the treatment
Random assignment: Using chance to place subjects in experiment groups
Observational study: A study that observes without assigning treatments
Experiment: A study where researchers assign treatments

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