AP Statistics

Sections: 1.Introduction  |  2. Data  |  3. Displaying Distributions   |  4. Inspecting Distributions  | 5. Time Plots  |  6. Measuring Center | 7. Measuring Spread  | 8. Linear Transformations |  9. Comparing Distributions

Time Plots Many variables are measured at intervals over time. We might, for example, measure the height of a growing child or the price of a stock at the end of each month. In these examples, our main interest is change over time. To display change over time, make a time plot. Time Plot A time plot of a variable plots each observation against the time at which it was measured. Always mark the time scale on the horizontal axis and the variable of interest on the vertical axis. If there are not too many points, connecting the points by lines helps show the pattern of changes over time. When you examine a time plot, look once again for an overall pattern and for strong deviations from the pattern. One common overall pattern is a trend, a long-term upward or downward movement over time. A pattern that repeats itself at regular time intervals is known as seasonal variation. The next example illustrates both these patterns. A Time Plot of International Piracy Piracy is defined as any illegal act of violence, detention, or any act of depredation committed for private ends against a ship on the high seas or otherwise outside of the jurisdiction of a state. Incidents of armed robbery against ships, similar acts which happen while a ship is within the territorial waters of a state, have also been included. The figure below provides the time series plot on the monthly number of incidents of international piracy, from 1995 to the present. Whereas the 1995-1997 period appeared to have maintained an overall level of approximately 20 incidents per month, the subsequent period seems less uniform. How much of this is attributable to seasonality? How much is due to a shift in the underlying trend?  International Piracy As we look at the period between January 99 and January 01 we see a general rise in International Piracy. After that period the trend levels off. When observations on a variable are taken over time, make a time plot that graphs time horizontally and the values of the variable vertically. A time plot can reveal trends, seasonal variations, or other changes over time.

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