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Cc�mprchcnsi��c Ncighbuurh�iud Sw�iics: Charactcriiing Dcclii�c Timeliness and frequency of the data. Another issue to consider is the timeliness and frequency of the data. Up-to-date data may not always be possible to get. U.S. Census data, for example, are collected only once every ten years. While the Census Bureau does provide revised estimates of population characteristics between decennial censuses, these estimates are not available for small levels of geography such as neighbourhoods. Therefore, to compare populations across cities, it may be possible to use very recent data, but neighbourhood comparisons may have to rely on older information. Geo�raphy of the data. Data from some sources may be available for small levels of geography (blocks, Census tracts, dissemination areas), while other data may only be provided for larger areas (cities, counties, states). Survey data are especially difficult to get for small areas, because to be able to produce accurate estimates one has to have many observations in the survey sample. Consequently, sources such as the Cunent Population Survey (CPS) or the American Housing Survey (AHS) in the United States cannot be used for very small areas. 4.1.2. Primary Data While the wealth of information can be obtained from existing sources, some areas are not covered by any pre-existing data sources. In this case one may need to collect original data through surveys or other methods. With original data, refened to as primary data, there is almost complete control over how the information is collected. The researcher specifies the overall goal of the data collection effort, what questions are asked, who is included in the sample, and so forth. There are a variety of ways of primary data collection. Surveys are one of the most common methods. At the other end of the spectrum are more open forms of data gathering, such as focus groups. All primary data collection methods have their own special technique and procedures as well as particular ways of analysing the resulting information. Primary data have a number of advantages over secondary data. Original data can be tailored exactly to the program or community's needs. As a result, original data can provide the most critical indicators by conforming more precisely to the chosen objectives. Furthermore, the community is more likely to support a program with original data that characterise their unique situation. With sufficient resources, primary data can be made as precise as needed (such as to produce estimates for individual neighbourhoods) and can be collected as often as needed (Tatian 2000). The major disadvantage is the cost and effort that are required to collect primary data. This is especially true for neighbourhood-level data, where a large number of observations are needed to obtain sufficiently accurate estimates for small areas. Collecting new data can absorb valuable resources that might otherwise be devoted to other efforts. Finally, certain primary data collection methods require technical expertise or resources that are not readily available in some communities (ibid.). —� —. 29