Sometimes you want to assign your newly computed variable a system missing value (which is a . in spss database). Here is the command for that:
COMPUTE temp = $sysmis.
(this syntax will create a variable called temp which will initially have all values set as missing)
For a conditional function:
IF sysmis(v1) v2=$sysmis.
You can also use missing values in RECODE.
RECODE [your command] (ELSE=sysmis).
Also read the UCLA SPSS page and CDC page on handling of missing data.
Thursday, 3 November 2011
Tuesday, 1 November 2011
PURPOSIVE SAMPLING - Subjects are selected because of some characteristic. Patton (1990) has proposed the following cases of purposive sampling. Purposive sampling is popular in qualitative research.
- Extreme or Deviant Case - Learning from highly unusual manifestations of the phenomenon of interest, such as outstanding success/notable failures, top of the class/dropouts, exotic events, crises.
- Intensity - Information-rich cases that manifest the phenomenon intensely, but not extremely, such as good students/poor students, above average/below average.
- Maximum Variation - Purposefully picking a wide range of variation on dimensions of interest...documents unique or diverse variations that have emerged in adapting to different conditions. Identifies important common patterns that cut across variations.
- Homogeneous - Focuses, reduces variation, simplifies analysis, facilitates group interviewing.
- Typical Case - Illustrates or highlights what is typical, normal, average.
- Stratified Purposeful - Illustrates characteristics of particular subgroups of interest; facilitates comparisons.
- Critical Case - Permits logical generalization and maximum application of information to other cases because if it's true of this once case it's likely to be true of all other cases.
- Snowball or Chain - Identifies cases of interest from people who know people who know people who know what cases are information-rich, that is, good examples for study, good interview subjects.
- Criterion - Picking all cases that meet some criterion, such as all children abused in a treatment facility. Quality assurance.
- Theory-Based or Operational Construct - Finding manifestations of a theoretical construct of interest so as to elaborate and examine the construct.
- Confirming or Disconfirming - Elaborating and deepening initial analysis, seeking exceptions, testing variation.
- Opportunistic - Following new leads during fieldwork, taking advantage of the unexpected, flexibility.
- Random Purposeful - (still small sample size) Adds credibility to sample when potential purposeful sample is larger than one can handle. Reduces judgment within a purposeful category. (Not for generalizations or representativeness.)
- Politically Important Cases - Attracts attention to the study (or avoids attracting undesired attention by purposefully eliminating from the sample politically sensitive cases).
- Convenience - Saves time, money, and effort. Poorest rational; lowest credibility. Yields information-poor cases.
- Combination or Mixed Purposeful - Triangulation, flexibility, meets multiple interests and needs. (Patton, 1990)