He also noted a problem with the Regional Model Life Tables; they were based on countries with higher female mortality, which would bias Coale's numbers of missing women downwards.
Furthermore, Klasen and Wink noted that both Sen's and Coale's methodologies were flawed because Sen and Coale assume that optimal sex ratios are constant across time and space, which they are often not.
Even within countries, the prevalence of missing women can vary drastically.
Das Gupta observed that the preference for boys and the resulting shortage of girls was more pronounced in the more highly developed Haryana and Punjab regions of India than in poorer areas.
Using the number, he then arrived at an estimate of 60 million missing women, much lower than Sen's original estimate.For example, a 2005 study estimated that over 90 million females were "missing" from the expected population in Afghanistan, Bangladesh, China, India, Pakistan, South Korea and Taiwan alone.On the other hand, Guilmoto in his 2010 report uses recent data (except for Pakistan), and estimates a much lower number of missing girls in Asian and non-Asian countries, but notes that the higher sex ratios in numerous countries have created a gender gap - shortage of girls - in the 0–19 age group.Overall they found trends that showed that while West Asia, North Africa and most of South Asia saw more equal sex ratios, China's and South Korea's ratios worsened.In fact, Klasen and Wink noted that China was responsible for 80% of the rise in missing women from between 19.