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Before starting any study or collecting any data, a researcher should ensure that data management, including data collection, data “cleaning'and analysis, is an integral part of the study design. Methods for guaranteeing that data are collected and managed in a reliable way should be implemented before the study begins. However, whether or not this ideal is achieved, it is always important to examine data in an exploratory, descrip-tive, manner. Such examination should include graphical displays as well as descriptive summaries using frequencies, means, and other summary statistics. These descriptive analyses serve at least two major purposes: 1. Errors in the data can be detected and corrected before too much time has been wasted on analyses giving incorrect results. This process is known as data cleaning, and is often carried out most effectively using graphical techniques. For example, a child's age …