Table 4.

Linear mixed effects regression model evaluating the relationship between monthly copayment and medication adherence (n = 12389 ^)*

Patient or Claim Characteristic †	Adherence, expressed as absolute percent change in proportion of days covered	P value
$$\sqrt[4]{\mathrm{Copayment}\mathrm{in}\mathrm{dollars}}$$	-8%	<0.0001
Medication class – inhaled steroid	-49%	<0.0001
Zip code-based estimates of patients’ annual income‡
$45 k – $75 k	6%	<0.0001
> $75 k	8%	<0.0001
Medication dosing frequency greater than once daily	-4%	<0.0001

^*Variables without association, and therefore not used as predictors in this model, are: patient age, gender, history of depression (as evidenced by prior use of antidepressants), and patient race (as estimated from zip code data).

^†The referent categories were as follows: angiotensin receptor blocker medication class, annual income < $45 k annually, and medication dosing less than or equal to once daily.

^‡Estimated based on zip code data. 336 claims were excluded from this analysis due to missing zip code data.