Descriptive Designs: Simple Time Series

1. Descriptive Designs: Simple Time Series Lani Pokrana Policy Analysis and Program Evaluation March 12, 2008

2. What is a Time Series Study? • A study that collects data at regular intervals • Examples: • Unemployment Rates • Crime Rates

3. Purpose • Establish a baseline measure • Describe changes over time • Keep track of trends • Forecast future trends

4. How to Conduct a Time Series Study • Collect data at regular intervals • Keep account of events to explain unexpected patterns • Present findings in a graph or table • Time is the implied independent variable (horizontal axis) • Dependent variable is along the vertical axis

5. Variations to consider • There are four variations to look for in a time series study: • Long-term trends • Cyclical variations • Seasonal variations • Irregular (or random) fluctuations

6. Long-Term Trends • General movement of a variable (upward or downward) over several years.

7. Cyclical Variations • Regularly occurring fluctuations within a long term trend • Last more than a year • Recur in one- to five-year intervals • A complete cycle is “peak to peak” or “valley to valley”

8. Seasonal Variations • Changes that occur within the course of a year. • Fluctuations traceable to weather or holidays • Recur year after year

9. Irregular (or random) Fluctuations • Variations not associated with long-term trends, cyclical variations, or seasonal variations • Nonrandom movement: conditions that can be identified and explain variation • Random movement: unexplained variations (often minor)

10. Example

11. Advantages • Communicates information quickly through graphs or tables • Easy to interpret • Useful for forecasting trends (short-term trends) • Helpful in evaluating the effectiveness of a policy

12. Disadvantage • Forecasts made for longer periods are less likely to be accurate

13. Case Study: One in 100: Behind Bars in America 2008 By: The PEW Center on the States

14. Purpose of Case • Launched in 2006 as an initiative of the Center, the Public Safety Performance Project seeks to help states advance fiscally sound, data-driven policies and practices in sentencing and corrections that protect public safety, hold offenders accountable, and control corrections costs.

15. Methodology • 2007-1/1/2008 prison population data collected through survey and communication with State Department of Corrections facilities • 1987-2006 prison population data collected from the Bureau of Justice Statistics • Total adult population count retrieved from the Census Bureau • State Corrections spending data retrieved from the National Association of State Budget Officers

16. Case Findings • Study projects a pattern of steady expansion of the U.S. prison population.

17. Case Findings • The study finds that more than one in every 100 adults is now confined in an American jail or prison. • As of January 1, 2008: • American prison system held more than 2.3 million adults • China was second with 1.5 million people behind bars • Russia was third with 890,000 inmates

18. Case Findings • The study notes that total spending on corrections continues to increase.

19. Case Findings • Total state spending on corrections—including bonds and federal contributions—topped $49 billion last year, up from $12 billion in 1987. By 2011, continued prison growth is expected to cost states an additional $25 billion. • Health care and staff overtime are the principal cost drivers

20. Results • Study indicates that much of the growth within prisons is a result of changing correctional policies and practices. • Lawmakers are learning that current prison growth is not driven primarily by a parallel increase in crime, or a corresponding surge in the population at large. Rather it flows principally from a wave of policy choices. • With the time series projections, policy makers are encouraged to consider other strategies for community corrections.

21. Questions?