My experience is that most businesses sales have a seasonal component to them. With some basic statistics you can determine your seasonality factors and trend as well as isolate irregularities that you can then research to determine the cause. Generally, my niche follows a typical retail cycle with November to March generally the highest sale months and drops starting in May until September. We may still have 20% growth in those months after seasonally adjusting. We generally use a basic regression analysis based on a rolling 12 month figure. I like to use 3 years of data if available. Almost all the non seasonal changes I can attribute to product fads, new products we added, and marketing changes.