Stock Returns
ROW/COL B C D E F G H I
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4 As you know many things in this world are normally distributed. This is very convenient for statistical analysis.
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6 Unfortunately stock returns are not normally distributed. You can described the reason in a variety of ways but the basic reason is that stock prices cannot be negative.
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8 However, there is a fair amount of evidence that stock returns are approximately normally distributed if you take the log (natural) of the return. 
9 To do this is fairly simple.
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12 Below is the data for IBM from Feb, 1998 to May, 2003
13 The blue numbers are the ln returns which are in general the ones you want to use for analysis such as computing Beta's and it is also the data needed to implement the Option Pricing models.
14 Date Adjusted Close Cash Dividend 1+ Return = (Pt+Dt)/Pt-1 ln(1+Return)
15
=(+C17+D17)/C16
16 Feb-98 51.81
17 Mar-98 51.53 0.99460 -0.542% <=LN(E17)
18 Apr-98 57.48 1.11547 10.927%
19 May-98 58.29 1.01409 1.399%
20 Jun-98 56.96 0.97718 -2.308%
21 Jul-98 65.73 1.15397 14.321%
22 Aug-98 55.87 0.84999 -16.253%
23 Sep-98 63.75 1.14104 13.194%
24 Oct-98 73.67 1.15561 14.463%
25 Nov-98 81.92 1.11199 10.615%
26 Dec-98 91.47 1.11658 11.027%
27 Jan-99 90.91 0.99388 -0.614%
28 Feb-99 84.21 0.92630 -7.656%
29 Mar-99 87.93 1.04418 4.323%
30 Apr-99 103.78 1.18026 16.573%
31 May-99 115.09 1.10898 10.344%
32 Jun-99 128.24 1.11426 10.819%
33 Jul-99 124.71 0.97247 -2.791%
34 Aug-99 123.59 0.99102 -0.902%
35 Sep-99 120.06 0.97144 -2.898%
36 Oct-99 97.48 0.81193 -20.834%
37 Nov-99 102.26 1.04904 4.787%
38 Dec-99 107.03 1.04665 4.559%
39 Jan-00 111.37 1.04055 3.975%
40 Feb-00 101.95 0.91542 -8.838%
41 Mar-00 117.45 1.15204 14.153%
42 Apr-00 110.63 0.94193 -5.982%
43 May-00 106.47 0.96240 -3.833%
44 Jun-00 108.71 1.02104 2.082%
45 Jul-00 111.37 1.02447 2.417%
46 Aug-00 130.99 1.17617 16.226%
47 Sep-00 111.74 0.85304 -15.895%
48 Oct-00 97.73 0.87462 -13.397%
49 Nov-00 92.77 0.94925 -5.209%
50 Dec-00 84.34 0.90913 -9.527%
51 Jan-01 111.13 1.31764 27.584%
52 Feb-01 99.12 0.89193 -11.437%
53 Mar-01 95.43 0.96277 -3.794%
54 Apr-01 114.24 1.19711 17.991%
55 May-01 110.93 0.97103 -2.940%
56 Jun-01 112.61 1.01514 1.503%
57 Jul-01 104.39 0.92700 -7.580%
58 Aug-01 99.17 0.95000 -5.130%
59 Sep-01 91 0.91762 -8.598%
60 Oct-01 107.23 1.17835 16.412%
61 Nov-01 114.69 1.06957 6.726%
62 Dec-01 120.02 1.04647 4.543%
63 Jan-02 107.05 0.89193 -11.436%
64 Feb-02 97.36 0.90948 -9.488%
65 Mar-02 103.19 1.05988 5.816%
66 Apr-02 83.11 0.80541 -21.641%
67 May-02 79.82 0.96041 -4.039%
68 Jun-02 71.44 0.89501 -11.092%
69 Jul-02 69.85 0.97774 -2.251%
70 Aug-02 74.96 0.15 1.07530 7.260%
71 Sep-02 57.98 0.77348 -25.686%
72 Oct-02 78.5 1.35392 30.300%
73 Nov-02 86.59 0.15 1.10497 9.982%
74 Dec-02 77.21 0.89167 -11.466%
75 Jan-03 77.91 1.00907 0.903%
76 Feb-03 77.81 0.15 1.00064 0.064%
77 Mar-03 78.29 1.00617 0.615%
78 Apr-03 84.74 1.08239 7.917%
79 May-03 86.18 1.01699 1.685%
80
81 Monthly Average Return = 0.817% <=AVERAGE(F17:F79)
82 Monthly Standard Deviation = 11.317% <=STDEV(F17:F79)
83
84 Annual Average Return = 9.800% <=(F81)*12
85 Annual Standard Deviation = 39.203% <=F82*(12)^0.5
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