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Lesson 3

Footwear Evidence

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Footprint – the trace everyone left

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Footprint – the trace everyone left

Source: SKH Lam Woo Memorial Sec. School“CSI Case study” (2010)

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Footprint – the trace everyone left

• YouTube - Leave No Trace - Footprint Evidence

(http://www.youtube.com/watch?v=qlmZZoLGMpM)

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Important features in footprints

important features significance

e.g. Size of footprint Real size of suspect’s foot approximate height of suspect

1 Shape of footprint Sex of the suspect2 Type and brand

(e.g. hiking shoes)Suspect’s hobbies

3 Depth of footprint on soft material

Weight of suspect

4 wear patterns and specific damages

Walking habit of suspect

5 trace materials, such as soil,

tar, rocks, and paint

Places where suspect had visited

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Discovery of footprint by oblique light

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Taking photo of

a footwear impression

a1a2

Case 1 Case 2

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Taking photo of a footwear impression

The ratio in case 2 is very close to that of the real shoe while a significant discrepancy appears in case 1. Case 1 is a wrong method since any angle other than 90o in taking the photo will cause distortion of the image. The size and shape of the shoe may be misinterpreted.

Correct Incorrect

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Examples of 2D and 3D footprints :

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Methods used in collecting footwear impressions

• (1) Electrostatic lifting

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Methods used in collecting footwear impressions

• (1) Electrostatic lifting

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Methods used in collecting footwear impressions

• (2) Casting

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Individual characteristics on a footprint

Figure 1 Figure 2

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outcomespossibleofnumbertotal

successofwaysofnumber

3200

1

8080

12

p

85333

1

808080

1233

p

Number of Characteristics

Probability of Occurrence p

1 1 / 80

2 1 / 3 200

3 1 / 85 333

Probability =

Number of Characteristics

Probability of Occurrence

1 1 / 10 000

2 1 / 50 000 000

3 1 / 166 666 666 667

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Individual characteristics on a footprint

• It is highly unlikely that two different shoes would share all the same identifying characteristics. If 2 -3 characteristics share between the footprint in a crime scene and the suspect’s shoe, it would be a strong evidence that the suspect had been to the crime scene. Otherwise, this may eliminate the suspect as having been there.

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Want to knows more ?

In probability theory, to say that two events are independent means that the occurrence of one event makes it neither more nor less probable that the other occurs.

For example:

The event of getting a 6 the first time a die is rolled and the event of getting a 6 the second time are independent.

If two cards are drawn with replacement from a deck of cards, the event of drawing a red card on the first trial and that of drawing a red card on the second trial are independent.

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Measurement of human gait at crime scene:

• Sex , • height of the thief , • direction of movement,• walking or running, • special walking habit,• carrying load or not, • drunken or under medication (if the gait shows instability)

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Measurement of human gait at crime scene :

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Measurement of human gait at crime scene :

(1) If footprints are deep and the pace is long, rapid movement is apparent. Long strides and deep prints with toe prints deeper than heel prints indicate running (A)

(2) Prints that are deep, short, and widely spaced, with signs of scuffing or shuffling indicate the person is carrying a heavy load (B).

(3) Persons walking backward (C) have a short, irregular stride. The prints have an unnaturally deep toe, and soil is displaced in the direction of movement.

(4) To determine the sex (D,E), we should study the size and position of the footprints. Women tend to be pigeon-toed, while men walk with their feet straight ahead or pointed slightly to the outside. Prints left by women are usually smaller and the stride is usually shorter than prints left by men.