Bias in Measurement: Effects, Treatment, and Consequences

1. Nov-2009 A bias is a bias is a bias By Dietmar Stöckl STT Consulting

2. A bias … • VIM 4.20 (5.25)# • instrumental bias = • average of replicate indications • minus a reference quantity value • … is a bias is a bias • http://en.wikipedia.org/wiki/Rose_is_a_rose_is_a_rose_is_a_rose • #http://www.bipm.org/en/publications/guides/vim.html Tin Can 11 - A Bias is a Bias is a Bias

3. Bias – Past & Present • There was (1) and there is (2) a tendency to put bias under the square root. • This was and this is wrong (3)! • This is NOT GUM! • When you HAVE a bias, TREAT it as a bias! • 1 Harris EK. Arch Pathol Lab Med 1988;112:416-20. • 2 http://www.westgard.com/rilibak-2.htm • 3 Thienpont LM. Clin Chem 2008;54:1587-8. Tin Can 11 - A Bias is a Bias is a Bias

4. Treatment of bias • 4 possibilities • ─neglect it (when small) • ─correct it • ─report it • ─put it into total error calculations • (in principle, this is the same as "report it") Tin Can 11 - A Bias is a Bias is a Bias

5. Bias & GUM# • GUM unequivocally states that bias has to be corrected. Only in EXCEPTIONAL cases (F.2.4.5), bias may be REPORTED, but not taken under the square root. • In principle, we then come to the same numbers as the total error concept, just the other way round: • TE = Bias + z x s; GUM = k x s + Bias. • The example given in GUM, however, does not exactly cover the situation in the clinical laboratory. Therefore, there always will be a point of discussion there. • #http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf Tin Can 11 - A Bias is a Bias is a Bias

6. Nature of bias & Consequences thereof • Bias is a location measure, it will offset your whole population. • If you fail to recognize that as analytical chemist, you will do a bad job. • Accreditation, regulation, are other things. • You should resist them when they ask you to do the wrong things. Tin Can 11 - A Bias is a Bias is a Bias

7. In the end • Media bias Tin Can 11 - A Bias is a Bias is a Bias