Portfolios and riskBeginner

Common investing mistakes

The most costly errors are usually process errors: seeking confirmation, confusing fall with value, ignoring size, and changing the thesis after the result.

3 min read Practical guide

By the end

You will implement simple controls before, during and after each decision.

Biases before buying

FOMO pushes to act without price; fixed anchor maximums or targets; confirmation selects only favorable evidence. Force yourself to write the best counterargument and consult original sources.

Analysis errors

A story without a box, an isolated ratio and comparables chosen at convenience produce false certainty. Do not replace missing data with zero. Separate facts, estimates and opinions.

Example

“It has fallen 60%, it must return” anchors the maximum. The right question is how much it is worth with current profits and balance sheet.

Portfolio errors

Accidental concentration, necessary money, and unlimited averaging turn normal uncertainty into permanent damage. Define size and liquidity before falling in love with the thesis.

Decision diary

Records available information, expectation and review rule. Evaluate months later if the reasoning was good, even if the result was favorable. Improve one rule at a time.

Contrary thesis.

Primary source.

Adverse scenario.

Size.

Invalidation.

From data to a decision

Does the decision survive a bias checklist?

This guide cannot predict the next move on its own. It can build a conditional reading: what supports upside, what increases downside risk, and which evidence must appear before acting.

Favourable reading

There is contrary evidence, a base rate, and a written invalidation rule.

Adverse reading

You only seek confirmation, anchor to the high, or chase performance.

Required confirmation

Write the strongest reason not to buy and what would change your mind.

Reasoned example

A stock returning to its old high is not a law; the prior high does not prove fair value.

Applied workshop

Turn the explanation into a process

Follow these steps in order and keep the result, so you can repeat the analysis and identify what changed your decision.

1Use a pre-decision checklist requiring thesis, alternative, risk, valuation and invalidation.
2Actively seek contrary evidence and a base rate before accepting an attractive story.
3Separate outcome from process: a gain can result from a poor but lucky decision.
4Keep a journal and review patterns quarterly without rewriting prior beliefs after the result.

Review questions

Are you trying to confirm or to test?
Does your purchase price affect future value?
What would you do if the position did not exist?

Worked case

Audit without looking at the result

A purchase based on rumor earns 40%; another with a documented thesis loses 10% due to a foreseen risk. The result does not convert the first into a good process nor the second into an automatic error.

Evaluates source, scenarios, size and execution using only information available at that time. This is how you separate skill from luck.

Decision rule

Improve repeatable rules; Don't reward a bad decision because it worked once.

Put it into practice

Audit a past decision without using later information: Identify what you knew, what you assumed, and what rule would have reduced the error.