I have read three other threads about employee stock purchase plans (I get to buy company stock at a 15% discount) and come to the conclusion that I should sell most of my shares immediately after they are posted to my brokerage account.
My question is, if thousands of employees have the same plan as me and sell the stock on the same day four times a year, will the share price dip? Would it be beneficial, on average, to wait a certain amount of time (days? weeks? months?) before selling?
Usually the amount of the ESPP stocks is very small compared to the overall volume of the trading, so it shouldn't matter. But check if for your company it not so (look at the stock history for the previous ESPP dates, and volumes).
An instant 15% profit sounds good to me, so you can't go wrong selling as soon as you are able. Here are a couple other considerations:
Tax implications: When you sell the stock, you have to pay taxes on the profit (including that 15% discount). The tax rate you pay is based on how long you wait to sell it. If you wait a certain amount of time (usually 2 years, but it will depend on your specific tax codes) before you sell, you could be subject to lower tax rates on that profit. See here for a more detailed description. This might only apply if you're in the US.
Since you work for the company, you may be privy to a bit more information about how the company is run and how likely it is to grow. As such, if you feel like the company is headed in the right direction, you may want to hold on the the stock for a while.
I am generally wary of being significantly invested in the company you work for. If the company goes south, then the stock price will obviously drop, but you'll also be at risk to be laid off. As such you're exposed much more risk than investing in other companies. This is a good argument to sell the stock and take the 15% profit.*
* - I realize your question wasn't really about whether to sell the stock, but more for when, but I felt this was relevant nonetheless.
It depends on how the program is run. If the company runs the program out of treasury stock (shares that are authorized, but not issued), then there aren't any shares being purchased on the open market. Because of that, the share price wouldn't be affected.
If you look in your employer's annual report, you will probably find how the program is run and how many shares are issued annually under that program. By comparing that to the daily trading volume of the company's stock you can gauge whether there's any likelihood of the share price being affected by the employee purchases. That is, of course, if shares are being purchased on the open market.
For example, here is Books-A-Million's program, as described in their 2011 annual report:
Employee Stock Purchase Plan The Company maintains an employee stock purchase plan under which shares of the Company’s common stock are reserved for purchase by employees at 85% of the fair market value of the common stock at the lower of the market value for the Company’s stock as of the beginning of the fiscal year or the end of the fiscal year. On May 20, 2010, the stockholders of the Company approved an additional 200,000 shares available for issuance under the plan, bringing the aggregate number of shares that may be awarded to 600,000. Of the total reserved shares, 391,987, 373,432 and 289,031 shares have been purchased as of January 29, 2011, January 30, 2010 and January 31, 2009, respectively.
This describes an instance of the employee purchase program being run from unissued stock, not open market purchases. From it, we can tell 18,555 shares were issued during the past fiscal year. As their average daily volume is ~40,000 shares, if the program were run from a single open market purchase, it would have potential to "move the market". One would think, though, that a company running it from open market purchases would spread the purchases over a period of time to avoid running up the price on themselves.
