Decision theorist Howard Raiffa  introduces useful distinctions among three approaches to the tahlil of decisions. Normative tahlil is concerned with the rational solution to the decision problem.
It defines the ideal that actual decisions should strive to approximate. Descriptive tahlil is concerned with the manner in which real people actually make decisions. Prescriptive tahlil isconcerned with practical advice and help that people could use to make more rational decisions.
Financial advising is a prescriptive activity whose main objective shouldbe to guide investors to make decisions that best serve their interests. To adviseeffectively, advisors must be guided by an accurate picture of the cognitive and emotional weaknesses of investors that relate to making investment decisions:
their occasionally faulty assessment of their own interests and true wishes, the relevant facts that they tend to ignore, and the limits of their ability to accept advice and to live with the decisions they make.
Our article sketches some parts of that picture, as they have emerged from research on judgment, decisionmaking and regret over the last three decades.
The biases of judgment and decision making have sometimes been called cognitive illusions. Like visual illusions, the mistakes of intuitive reasoning are not easily eliminated.
Consider the example of Exhibit 1.
Although you can use a ruler to convince yourself
that the two horizontal lines are of equal length,
you will continue to see the second line as much longer than the other.
Merely learning about illusions does not eliminate them.