This was only a two marker and therefore requires one similarity and one difference; the tricky aspect here will be ensuring that each is in sufficient depth to gain the mark.
The following information may be helpful;
Difference: The ICD is published by the WHO, a global health agency whose primary mission is to improve public health, and is an international classification system for all diseases, relating to both physical and mental well being whereas the DSM is only used for the diagnosis of mental disorders and is published by the American Psychiatric Association.
Similarity: Both manuals take a descriptive approach to diagnosis, including sets of symptoms that may appear in a person with each condition and a psychiatrist may or may not reach a diagnosis by ruling out those disorders that include symptoms that the person is not displaying, gradually narrowing down the diagnosis to the one that fits their experience best.
Difference:“The WHO’s primary objective is to help countries to reduce the disease burden of mental disorders and the ICD’s development is global, multidisciplinary and multilingual whereas the DSM is used primarily by U.S. psychiatrists” (APA, 2009)
Difference: “While the ICD is distributed as broadly as possible at a very low cost, with substantial discounts to low-income countries, and available free on the Internet; the DSM generates a very substantial portion of the American Psychiatric Association’s revenue, not only from sales of the book itself, but also from related products and copyright permissions for books and scientific articles” (APA, 2009)