How common is complete heterochromia? (One iris is a different color from the other)
Are there any statistics available for this? If no, why not?
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Note: not my area of expertise
First of all, heterochromia can be congenital (found at birth) or acquired, and from your question it is not clear if you are asking about one specific type or the overall frequency (if so, at what age?). Since acquired heterochromia is often due to injury or disease, the overall frequency is likely to increase with age. Also see answers to the question "Is it biologically possible for an adult's eye color to change?" for more on reasons for acquired heterochromia.
According to an Austrian study (Stelzer, 1979), the overall frequency in the Vienna population was 0.26% (males: 0.16; females: 0.37). However, Medicine.net reports a frequency of 0.6% for only congenial heterochromia, but this number lacks a clear reference. A thorough review of different reasons for heterochomia can be found in Rennie (2012).