Any attempt to understand and analyze the state of world Jewry faces several challenges from the start.
First, outside the Jewish-majority State of Israel, which has all the attributes of a nation—census data, economic reports, global rankings, security assessments, and political barometers—Jews live in scores of other countries around the world where such attributes are usually lacking, making it difficult to collect information and reach conclusions about their Jewish communities.
Second, in the country with the largest concentration of Jews outside Israel, the United States, census guidelines prohibit questions about religion, which means that data collection and assessment are pursued by private – scholarly or communal – sources, if at all.
Third, there is no universally accepted definition of who is a Jew. Depending on the community, criteria can range from a very open understanding of Jewish identity – e.g., anyone who considers herself or himself a Jew, or anyone who has any Jewish ancestry – to much more restrictive definitions – e.g., only those who have a Jewish mother or those who have been converted to Judaism by certain rabbis and not others.