This is a very important question, when companies are looking to and are considering working with outsourced development companies to work with them. Stats prove that over 90% of development work that companies undertake are outsourced because they are looking additional professional resources and expertise.
Last year my colleague Peter Loudreaj appeared on a TV show in Norway hosted by Heidi Frost Eriksen the CEO of GrepS (www.GrepS.com) to answer this question on behalf of GrepS and the audience.
If of interest, please find enclosed a recording of that show.
https://www.youtube.com/watch?v=dXKgUUjUTuo&feature=youtu.be