- Hidden Markov models (HMMs) are statistical models where the system is assumed to be a Markov process with hidden states. Each state has a number of possible transitions to other states, each with an assigned probability. - There are three main issues in HMMs: model evaluation, decoding the most probable path, and model training. - HMMs have applications in areas like speech recognition, gesture recognition, language modeling, and video analysis.

2. CONTENTS
• Introduction
• Markov Model
• Hidden Markov model (HMM)
• Three central issues of HMM
– Model evaluation
– Most probable path decoding
– Model training
• Application Areas of HMM
• References

3. Hidden Markov Models
 Hidden Markow Models:
– A hidden Markov model (HMM) is a statistical
model,in which the system being modeled is
assumed to be a Markov process (Memoryless
process: its future and past are independent )
with hidden states.

4. Hidden Markov Models
 Hidden Markow Models:
– Has a set of states each of which has limited
number of transitions and emissions,
– Each transition between states has an
assisgned probability,
– Each model strarts from start state and ends
in end state,

5. Hidden Markov Models

6. Hidden Markov Models
 Markow Models :
 Talk about weather,
 Assume there are three types of weather:
– Sunny,
– Rainy,
– Foggy.

7. Markov Models
 Weather prediction is about the what would be the weather
tomorrow,
– Based on the observations on the past.

8. Markov Models
 Weather at day n is
– qn depends on the known weathers of the past
days (qn-1, qn-2,…)
},,{ foggyrainysunnyqn∈

9. Markov Models
 We want to find that:
– means given the past weathers what is the
probability of any possible weather of today.

10. Markov Models
 Markow Models:
 For example:
 if we knew the weather for last three days was:
 the probability that tomorrow would be is:
P(q4 = | q3 = , q2 = , q1 = )

11. Markov Models
 Markow Models and Assumption (cont.):
– Therefore, make a simplifying assumption Markov
assumption:
 For sequence:
 the weather of tomorrow only depends on today
(first order Markov model)

12. Markov Models
 Markow Models and Assumption (cont.):
Examples:
 HMM:

13. Markov Models
 Markow Models and Assumption (cont.):
Examples:
 If the weather yesterday was rainy and today is foggy
what is the probability that tomorrow it will be sunny?

14. Markov Models
 Markow Models and Assumption (cont.):
– Examples:
 If the weather yesterday was rainy and today is foggy
what is the probability that tomorrow it will be sunny?
Markov assumption

15. Hidden Markov Models
 Hidden Markov Models (HMMs):
– What is HMM:
 Suppose that you are locked in a room for several days,
 you try to predict the weather outside,
 The only piece of evidence you have is whether the
person who comes into the room bringing your daily
meal is carrying an umbrella or not.

16. Hidden Markov Models
 Hidden Markov Models (HMMs):
– What is HMM (cont.):
 assume probabilities as seen in the table:

17. Hidden Markov Models
 Hidden Markov Models (HMMs):
– What is HMM (cont.):
 Finding the probability of a certain weather
 is based on the observations xi:
},,{ foggyrainysunnyqn∈

18. Hidden Markov Models
 Hidden Markov Models (HMMs):
– What is HMM (cont.):
 Using Bayes rule:
 For n days:

19. Hidden Markov Models
 Hidden Markov Models (HMMs):
– Examples:
 Suppose the day you were locked in it was sunny. The
next day, the caretaker carried an umbrella into the
room.
 You would like to know, what the weather was like on
this second day.

20. 20
Discrete Markov Processes
(Markov Chains)

21. 21
Hiddden Markov Models

22. 22
Hidden Markov Models

23. 23
Hidden Markov Models

24. 24
Hidden Markov Model Examples

25. 25
Hidden Markov Models

26. 26
Hidden Markov Models

27. 27
Hidden Markov Models

28. 28
Three Fundamental Problems for
HMMs

29. 29
HMM Evaluation Problem

30. 30
HMM Evaluation Problem

31. 31
HMM Evaluation Problem

32. 32
HMM Evaluation Problem

33. 33
HMM Evaluation Problem

34. 34
HMM Decoding Problem

35. 35
HMM Decoding Problem

36. 36
HMM Decoding Problem

37. 37
HMM Learning Problem

38. 38
HMM Learning Problem

39. 39
HMM Learning Problem

40. 40
HMM Learning Problem

41. Application Areas of HMM
• On-line handwriting recognition
• Speech recognition
• Gesture recognition
• Language modeling
• Motion video analysis and tracking
• Stock price prediction
and many more….

42. References
 R.O. Duda, P.E. Hart, and D.G. Stork, Pattern Classification, New York: John Wiley, 2001.
 Selim Aksoy, “Pattern Recognition Course Materials”, Bilkent University, 2011.