# Hidden Markov Models in Finance, Markets & Trading

Written By
Written By
Dan Buckley
Dan Buckley is an US-based trader, consultant, and part-time writer with a background in macroeconomics and mathematical finance. He trades and writes about a variety of asset classes, including equities, fixed income, commodities, currencies, and interest rates. As a writer, his goal is to explain trading and finance concepts in levels of detail that could appeal to a range of audiences, from novice traders to those with more experienced backgrounds.
Updated

Hidden Markov Models (HMMs) are statistical models that represent systems with hidden states.

These models are useful in fields like speech recognition, bioinformatics, and financial analysis.

They capture the idea that the observed outputs of a system are the result of states that are not directly visible or “hidden.”

## Key Takeaways – Hidden Markov Models

• Hidden Markov Models (HMMs) identify unobservable market regimes (e.g., uptrend, downtrend, high/low volatility) using observable financial data.
• HMMs enable probabilistic inference of market states, and can help traders in their decision-making.
• They help in tactical trading by adjusting portfolios according to the inferred market conditions.

## Core Components of Hidden Markov Models

An HMM consists of the following elements:

### States

Hidden states represent the underlying process. The actual state isn’t visible.

### Observations

These are visible outputs or data points linked to the states.

### Transition Probabilities

These define the likelihood of transitioning from one state to another.

### Emission Probabilities

These express the likelihood of an observation being generated from a state.

### Initial State Probabilities

These indicate the probabilities of starting in each state.

## Operational Dynamics of Hidden Markov Models

The working of an HMM can be understood through three fundamental problems:

### Evaluation

Given the HMM parameters and a sequence of observations, determine the likelihood of the observation sequence.

### Decoding

Given the HMM and a sequence of observations, uncover the most likely sequence of hidden states.

### Learning

Adjust the model parameters to maximize the probability of the observation sequence.

## Applications of Hidden Markov Models

HMMs are versatile and find applications in numerous fields:

### Speech Recognition

They model the sequence of spoken words as hidden states and the audio features as observations.

### Bioinformatics

They help in gene prediction and protein modeling by treating the biological sequences as observations.

### Financial Markets

They can model market conditions as hidden states in risk management and trading/investment strategies.

For example, let’s say a trader is following a momentum strategy.

An HMM can be applied to detect different market trends such as bullish, bearish, or sideways market conditions.

Each state represents a regime with distinct asset behavior (momentum trading largely involves belief that what most recently happened will continue to happen).

The model uses observable market data like price movements and trading volumes to probabilistically infer the hidden state of the market.

For instance, in a detected bullish state, a trader might increase their allocation in equities, while in a bearish state, they might shift to less exposure or short.

### Risk Assessment

HMMs can help in assessing the risk profile of different assets under various market conditions.

This can enable a more nuanced understanding of how different assets contribute to overall portfolio risk in different regimes.

### Dynamic Asset Allocation

Once the market regimes are identified, a risk parity portfolio can be dynamically adjusted according to the predicted state.

For instance, in a high-volatility regime, the model might suggest increasing allocation to safer assets like short-term government bonds.

### Portfolio Diversification

By understanding the different market states and their impact on asset classes, HMMs can guide the diversification of a diversified portfolio to ensure that it is well-positioned across different market conditions.

## Limitations & Challenges

Some limitations of HMMs:

### Assumption of Independence

HMMs assume that each observation is independent. This might not always hold true.

### Scalability Issues

As the number of states increases, the complexity of the model can become computationally demanding.

### Hidden States

Determining the actual number and nature of hidden states can be challenging and often requires domain expertise.

### Complexity & Interpretation

The complexity of HMMs requires sophisticated statistical expertise and careful interpretation of the model’s outputs.

### Data-Driven

These models are heavily dependent on historical data, and their effectiveness is contingent on the quality and relevance of the data used.

## Conclusion

Hidden Markov Models are used for modeling systems with underlying hidden processes.

Understanding their limitations and applying them judiciously is important for obtaining meaningful results.