Also, you might have bought fewer shares of stock or securities than you sold. (hrblock.com)
By putting in the same amount of money each period, you will end up buying fewer shares when the market is up, and more when it is down. (money.stackexchange.com)
When you invest a set amount of your money each month, you buy fewer shares when the market is high and more shares when it's low. (investopedia.com)