When attempting to find effective SEO companies for potentially sourcing your work to, it is a good idea to try and make sure that the work that they have done for other customers has actually produced results that are beneficial. Simply landing clients and having them in the portfolio does not show effectiveness, as many companies are in the business of providing short contracts with no belief that they will be retained over time due to the fact that their services do not present results. If a company is not very effective, they are going to show a large amount of short term clients, due to the fact that they are not retained after the clients realize that their services are not producing the revenue that is necessary to keep them employed. A good seo company will pay for itself through the additional revenues gained from their efforts, and should be able to make you more money than they cost you.