title: Secondary data analysis of large data sets in urology: successes and errors to avoid.
abstract: OBJECTIVE
  Secondary data analysis is the use of data collected for research by someone other than the investigator. In the last several years there has been a dramatic increase in the number of these studies being published in urological journals and presented at urological meetings, especially involving secondary data analysis of large administrative data sets. Along with this expansion, skepticism for secondary data analysis studies has increased for many urologists.
  
  
  METHODS
  In this narrative review we discuss the types of large data sets that are commonly used for secondary data analysis in urology, and discuss the advantages and disadvantages of secondary data analysis. A literature search was performed to identify urological secondary data analysis studies published since 2008 using commonly used large data sets, and examples of high quality studies published in high impact journals are given. We outline an approach for performing a successful hypothesis or goal driven secondary data analysis study and highlight common errors to avoid.
  
  
  RESULTS
  More than 350 secondary data analysis studies using large data sets have been published on urological topics since 2008 with likely many more studies presented at meetings but never published. Nonhypothesis or goal driven studies have likely constituted some of these studies and have probably contributed to the increased skepticism of this type of research. However, many high quality, hypothesis driven studies addressing research questions that would have been difficult to conduct with other methods have been performed in the last few years.
  
  
  CONCLUSIONS
  Secondary data analysis is a powerful tool that can address questions which could not be adequately studied by another method. Knowledge of the limitations of secondary data analysis and of the data sets used is critical for a successful study. There are also important errors to avoid when planning and performing a secondary data analysis study. Investigators and the urological community need to strive to use secondary data analysis of large data sets appropriately to produce high quality studies that hopefully lead to improved patient outcomes.
authors.affiliation: Baylor College of Medicine and Texas Children's Hospital, Houston, Texas.
authors.forename: Bruce J
authors.initials: BJ
authors.lastname: Schlomer
authors.affiliation: University of California San Francisco, San Francisco, California.
authors.forename: Hillary L
authors.initials: HL
authors.lastname: Copp
mesh_terms: D035843:Biomedical Research; D003627:Data Interpretation, Statistical; D006801:Humans; D017063:Outcome Assessment, Health Care; D010506:Periodicals as Topic; D011643:Publishing / Q000706:statistics & numerical data; D014572:Urology / Q000706:statistics & numerical data*
keywords: outcome assessment; research design