Let’s start with the time adjustment.
Suppose, for the first half of the year, there were 1 million shares outstanding. On July 1, the company sold another million shares on the open market at a price of 50 dollars per share. For the second half, there were 2 million shares outstanding. It seems reasonable to calculate the average number of shares outstanding at
[math]\dfrac{1}{2} * 1000000 + \dfrac{1}{2} * 2000000 = 500000 + 1000000 = 1500000.[/math]
Suppose, however, there was a one for one stock split at opening of business on July 1. Each share in the first half of the year magically became two shares in the second half of the year. Now it seems reasonable at year end to say that there were 2 million of the current type of share outstanding for the entire year.
If there were shares issued under a rights issue, we have an intermediate situation.
What is a simple way but reasonable and consistent way to account for any situation?
What the accountants do is to weight the number of shares by “fair value“ per share and by time outstanding.
How to calculate the “fair value per share“ weight? Calculate the fair value of the company immediately before the transaction and divide by the number of shares outstanding immediately before the transaction. That gives a before-value in dollars per share, and lets us compute the fair value of the firm at that instant. Then calculate the fair value of the company immediately after the transaction and divide by the number of shares outstanding immediately after the transaction. That gives an after-value in dollars per share. Now take the ratio of the before-value over the after-value. Dollars per share in the numerator and in the denominator cancel out, and we are left with a pure number to get our weight.
Let’s see how this works out in the extreme cases.
We had a million shares and sold a million more at a price of $50 per share. What do the accountants say is fair value in this case? They say that we have objective evidence that the value of each share before the transaction occurred is at least 50 dollars per share. So we estimate the before-value at $50 per share, giving an estimated fair value for the firm of $50,000,000 before the transaction. But the transaction added $50,000,000 to the assets of the firm without any increase in liabilities. So the fair value per share after the transaction is
[math]\dfrac{50,000,000 + 50,000,000}{1,000,000+ 1,000,000} = \dfrac{100,000,000}{2,000,000} = 50 \text { dollars per share}[/math]
But the ratio of 50 / 50 is 1. So
[math]\dfrac{1}{2} * 1 * 1,000,000 + \dfrac{1}{2} * 2,000,000 = 500,000 + 1,000,000 = 1,500,000.[/math]
This gives us the common-sense result shown above.
Now let’s think about the stock split example: we went from a million to two million shares with an old stock price of 50 per share. The new shares were issued for nothing or zero. The value of the underlying firm has not changed. The only objective evidence of the fair value per share before the split is the market value per share before the split of $50. So the fair value per share after the split is
[math]\dfrac{50,000,000 + 0}{1,000,000 + 1,000,000} = \dfrac{50,000,000}{2,000,000} = 25 \text { dollars per share}[/math]
But the ratio of 50 over 25 is 2.
[math]\dfrac{1}{2} * 2 * 1,000,000 + \dfrac{1}{2} * 2,000,000 = 1,000,000 + 1,000,000 = 2,000,000.[/math]
Again, this gives us the common-sense result shown above. What the weighting does is to count shares before the transaction in terms of relative fair value to shares after the transaction.
Let’s see what we are doing in common sense terms. We know the market price per share before the transaction. So the fair value of the firm before the transaction is simply the price per share times the shares outstanding. The fair value of the firm after the transaction is the sum of the fair value of the firm before the transaction and the cash realized from the transaction. To get the new fair value per share, we simply divide the fair value of the firm after the transaction by the number of shares outstanding after the transaction. We compute our fair-value weight as the ratio of the before value per share over the after value per share. It is all a very common sense way to make a reasonable adjustment in a consistent way. Let’s apply it to the rights case.
We have a million shares outstanding on June 30 with a market price per share. Under a rights offering, an extra million shares are issued at a price of $45 per share. The old value of the firm was $50 million. The new value is
$50 million + $45 million = $95 million. There are now 2 million shares outstanding. That gives a new value per share of $47.50.
We say the shares outstanding before July 1 count as 50/47.50 as many shares as the shares after June 30.
It is just expressed in fancy words.