Which research technique must researchers be sure to employ to corroborate data obtained ...

Which research technique must researchers be sure to employ to corroborate data obtained unobtrusively?

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Estimate the standard error of the mean with the following sample of 37 scores on a 7- point scale of views on affirmative action (with 1 being the most in favor of affirmative action and 7 being the most opposed to affirmative action):
 
  7 1 4 6
  5 1 5 2
  4 2 5 3
  6 5 2 1
  6 7 1 5
  2 4 1 7
  1 3 3 2
  1 5 2
  3 2 4
  6 4 7
 
  What would be an ideal response?

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Using publicly published police records and the accompanying video or audio recordings of the police report to study crimes committed during the day or evening of the published account is an example of which type of unobtrusive method?
 
  A) Official documentary records
  B) Accretion
  C) Physical erosion
  D) Archival strategies