Google, Yahoo and MSN all provide both web user interfaces (WUIs) and application programming interfaces (APIs) to their collections. Whether building collections of resources or studying the search engines themselves, the search engines request that researchers use their APIs and not “scrape ” the WUIs. However, anecdotal evidence suggests the interfaces produce different results. We provide the first in depth quantitative analysis of the results produced by the Google, MSN and Yahoo API and WUI interfaces. We have queried both interfaces for five months and found significant discrepancies between the interfaces in several categories. In general, we found MSN to produce the most consistent results between their two interfaces. Our findings suggest that the API indexes are not older, but they are probably smaller for Google and Yahoo. We also examined how search results decay over time and built predictive models based on the observed decay rates. Based on our findings, it can take over a year for half of the top 10 results to a popular query to be replaced in Google and Yahoo; for MSN it may take only 2-3 months.