William, the owner of a small hardware store, likes to forecast sales to better prepare for seasonal fluctuations. He identifies as many target markets as possible and predicts the sales for each based on one-day sales at different times of the year. He then combines the predictions for the various segments to create a total sales forecast. Which sales forecasting method is William using?
a. Trend analysis
b. Regression analysis
c. Bottom-up forecasting
d. Top-down forecasting