#4. A brokerage house would like to be able to predict the number of trade executions per day and has decided to use the number of incoming phone calls as a predictor variable. Data were collected over a period of 35 days and are presented in the table below. Fit a linear model and find the standard error of the model (sigma). Round your answer to 3 decimal places.

Day	Calls	Executions
1	2591	417
2	2146	321
3	2185	362
4	2245	364
5	2600	442
6	2510	386
7	2394	370
8	2486	376
9	2483	463
10	2297	389
11	2106	302
12	2035	266
13	1936	339
14	1951	369
15	2292	403
16	2094	319
17	1897	306
18	2237	397
19	2328	365
20	2078	330
21	2134	312
22	2192	340
23	1965	339
24	2147	364
25	2015	295
26	2046	292
27	2073	379
28	2032	294
29	2108	329
30	1923	274
31	2069	326
32	2061	306
33	2010	352
34	1913	290
35	1904	283