A. Event Study
An event study is designed to examine market reactions to, and excess returns around specific information events. The information events can be market-wide, such as macro-economic announcements, or firm-specifc, such as earnings or dividend announcements.

Step 1: Identify the event

(1) The event to be studied is clearly identified, and the date on which the event was announced pinpointed.

Announcement Date


Step 2: Collecting Returns

(2) Once the event dates are known, returns are collected around these dates for each of the firms in the sample. In doing so, two decisions have to be made.

- how precisely the event date is known

- by how quickly information is reflected in


R-jn ............................. Rj0 ...............................R+jn


Return window: -n to +n


Rjt = Returns on firm j for day t (t = -n, ...,0, .... +n)

Step 3: Adjust for market performance and risk

(3) The returns, by period, around the announcement date, are adjusted for market performance and risk to arrive at excess returns for each firm in the sample. For instance, if the capital asset pricing model is used to control for risk -

Excess Return on day t = Return on day t - Beta * Return on market on day t

ER-jn ............................ ERj0 ............................ER+jn


Return window: -n to +n


ERjt = Excess Returns on firm j for day t (t = -n, ...,0, .... +n)

Step 4: Calculate the crosssectional average

(4) The excess returns, by day, are averaged across all firms in the sample and a standard error is computed.

Average excess return on day t=


N = Number of events in the event study

Step 5: Estimate the statistical significance

(5) The question of whether the excess returns around the announcement are different from zero is answered by estimating the t statistic for each n, by dividing the average excess return by the standard error -

T statistic for excess return on day t = Average Excess Return / Standard Error

If the t statistics are statistically significant, the event affects returns; the sign of the excess return determines whether the effect is positive or negative.

Illustration 1: Example of an event study - Effects of Option Listing on Stock prices

Step 1: The date on which the announcement that options would be listed on the Chicago Board of Options on a particular stock was collected.

Step 2: The prices of the underlying stock(j) were collected for each of the ten days prior to the option listing announcement date, the day of the announcement, and each of the ten days after.

Step 3: The returns on the stock (Rjt) were computed for each of these trading days.

Step 4: The beta for the stock (bj) was estimated using the returns from a time period outside the event window (using 100 trading days from before the event and 100 trading days after the event).

Step 5: The returns on the market index (Rmt) were computed for each of the 21 trading days.

Step 6: The excess returns were computed for each of the 21 trading days -

ERjt = Rjt - bj Rmt .......... t = -10,-9,-8,....,+8,+9,+10

The excess returns are cumulated for each trading day.

Step 7: The average and standard error of excess returns across all stocks with option listings were computed for each of the 21 trading days. The t statistics are computed using the averages and standard errors for each trading day.

Option Listing and Stock Returns: The Results
Trading Day
Average Excess



T Statistic

B. Portfolio Study

Steps in doing a portfolio study

(1) The variable on which firms will be classified is defined, using the investment strategy as a guide. This variable has to be observable, though it does not have to be numerical.

(2) The data on the variable is collected for every firm in the defined universe at the start of the testing period, and firms are classified into portfolios based upon the magnitude of the variable.

(3) The returns are collected for each firm in each portfolio for the testing period, and the returns for each portfolio are computed, generally assuming that the stocks are equally weighted.

(4) The beta of each portfolio is estimated, either by taking the average of the betas of the individual stocks in the portfolio or by regressing the portfolio's returns against market returns over a prior period.

(5) The excess returns earned by each portfolio are computed, with the standard error of these returns.

(6) There are a number of statistical tests available to check whether the average excess returns are, in fact, different across the portfolios.

(7) As a final test, the extreme portfolios can be matched against each other to see whether there are statistically significant differences across these portfolios.

Illustration 8.2: Example of a portfolio study - Price Earnings Ratios

Step 1: Using data on PE ratios from the end of 1987, firms on the New York Stock Exchange were classified into five groups, the first group consisting of stocks with the lowest PE ratios and the fifth group consisting of stocks with the highest PE ratios. Firms with negative price-earnings ratios were ignored.

Step 2: The returns on each portfolio were computed using data from 1988 to 1992. Stocks which went bankrupt or were delisted were assigned a return of -100%.

Step 3: The betas for each stock in each portfolio were computed using monthly returns from 1983 to 1987, and the average beta for each portfolio was estimated. The portfolios were assumed to be equally weighted.

Step 4: The returns on the market index was computed from 1988 to 1992.

Step 5: The excess returns on each portfolio were computed using data from 1988 to 1992. The following table summarizes the excess returns each year from 1988 to 1992 for each portfolio.

Table 8.2: Excess Returns from 1988 to 1992 for PE Ratio Portfolios

P/E Class 1988 1989 1990 1991 1992 1988-1992
Lowest 3.84% -0.83% 2.10% 6.68% 0.64% 2.61%
2 1.75% 2.26% 0.19% 1.09% 0.93% 1.46%
3 0.20% -3.16% -0.20% 0.17% 0.12% -0.59%
4 -1.25% -0.94% -0.65% -2.01% -0.48% -1.15%
Highest -1.74% -0.63% -1.44% -4.06% -1.25% -1.95%

The Cardinal Sins in testing Market Efficiency

1. Using 'anecdotal evidence' to support/reject an investment strategy: Anecdotal evidence is a double edged sword. It can be used to support or reject the same hypothesis. Since stock prices are noisy and all investment schemes (no matter how absurd) will succeed sometimes and fail at other times, there will always be cases where the scheme works or does not work.

2. Testing an investment strategy on the same data and time period from which it was extracted: This is the tool of choice for the unscrupulous investment advisor. An investment scheme is extracted from hundreds through an examination of the data for a particular time period. This investment scheme is then tested on the same time period, with predictable results. (The scheme does miraculously well and makes immense returns.)

An investment scheme should always be tested out on a time period different from the one it is extracted from or on a universe different from the one used to derive the scheme.

3. Choosing a biased universe, The universe is the sample on which the test is run. Since there are thousands of stocks that could be considered part of this universe, researchers often choose to use a smaller universe. When this choice is random, this does limited damage to the results of the study. If the choice is biased, it can provide results which are not true in the larger universe.

4. Failure to control for market performance: A failure to control for overall market performance can lead one to conclude that your investment scheme works just because it makes good returns (Most schemes will make good returns if the overall market does well; the question is did they make better returns than expected) or does not work just because it makes bad returns (Most schemes will do badly if the overall market performs poorly). It is crucial therefore that investment schemes control for market performance during the period of the test.

5. Failure to control for risk: A failure to control for risk leads to a bias towards accepting high-risk investment schemes and rejecting low-risk investment schemes, since the former should make higher returns than the market and the latter lower, without implying any excess returns.

Some lesser sins that can be a problem

1. Survival Bias: Most researchers start with a existing universe of publicly traded companies and working back through time to test investment strategies. This can create a subtle bias since it automatically eliminates firms that failed during the period, with obvious negative consequences for returns. If the investment scheme is particularly susceptible to picking firms that have high bankruptcy risk, this may lead to an 'overstatement' of returns on the scheme.

For example, assume that the investment scheme recommends investing in stocks which have very negative earnings, using the argument that these stocks are most likely to benefit from a turnaround. Some of the firms in this portfolio will go bankrupt, and a failure to consider these firms will overstate the returns from this strategy.

An Example of Survivor Bias

2. Not allowing for transactions Costs: Some investment schemes are more expensive than others because of transactions costs - execution fees, bid-ask spreads and price impact. A complete test will take these into account before it passes judgment on the strategy. This is easier said than done, because different investors have different transactions costs, and it is unclear which investor's trading cost schedule should be used in the test. Most researchers who ignore transactions costs argue that individual investors can decide for themselves, given their transactions costs, whether the excess returns justify the investment strategy.

3. Not allowing for difficulties in execution: Some strategies look good on paper but are difficult to execute in practice, either because of impediments to trading or because trading creates a price impact. Thus a strategy of investing in very small companies may seem to create excess returns on paper, but these excess returns may not exist in practice because the price impact is significant.

Many a slip between the cup and the lip ....