The uniform distribution is a continuous probcapacity circulation and also is concerned with occasions that are equally most likely to occur. When working out problems that have actually a unicreate distribution, be careful to note if the data is inclusive or exclusive.

You are watching: The shape of any uniform probability distribution is ____________.

Example 1

The information in the table listed below are 55 smiling times, in seconds, of an eight-week-old baby.


The sample mean = 11.49 and the sample traditional deviation = 6.23.

We will assume that the smiling times, in seconds, follow a uniform circulation between zero and 23 seconds, inclusive. This indicates that any type of smiling time from zero to and also including 23 secs is equally likely. The histogram that might be built from the sample is an empirical circulation that closely matches the theoretical unicreate distribution.

Let X = size, in secs, of an eight-week-old baby’s smile.

The notation for the unidevelop distribution is X ~ U(a, b) wright here a = the lowest worth of x and b = the highest worth of x.

The probcapability thickness function is displaystylef(x)=frac1b-a\ for axb.

For this instance, X ~ U(0, 23) and also displaystylef(x)=frac123-0\ for 0 ≤ X ≤ 23.

Formulas for the theoretical mean and typical deviation are displaystylemu=fraca+b2quad extandquadsigma=sqrtfrac(b-a)^212\

For this problem, the theoretical expect and conventional deviation are displaystylemu=frac0+232=11.50 ext secondsquad extandquadsigma=sqrtfrac(23-0)^212=6.64 ext seconds\

Notice that the theoretical expect and standard deviation are close to the sample intend and typical deviation in this instance.

Try It

The information that follow are the variety of passengers on 35 various charter fishing boats. The sample expect = 7.9 and also the sample typical deviation = 4.33. The data follow a unicreate circulation where all values in between and consisting of zero and 14 are equally most likely. State the values of a and b. Write the distribution in appropriate notation, and also calculate the theoretical mean and also standard deviation.


a is zero; b is 14; X ~ U (0, 14); μ = 7 passengers; σ = 4.04 passengers

Example 2

Refer to Example 1 What is the probability that a randomly chosen eight-week-old baby smiles in between 2 and also 18 seconds?Find the 90th percentile for an eight-week-old baby’s smiling time.Find the probcapacity that a random eight-week-old baby smiles more than 12 seconds knowing that the baby smiles even more than eight seconds.


Find P(2 displaystyleP{(2{Ninety percent of the smiling times autumn listed below the 90th percentile, k, so P(x P(x( extbase)( extheight)=0.90\displaystyle(k-0)(frac123)=0.90\k=(23)(0.90)=20.7\
conditional. You are asked to discover the probability that an eight-week-old baby smiles more than 12 seconds as soon as you currently know the baby has smiled for even more than eight secs.Find P(x > 12|x > 8) There are 2 methods to do the problem.For the initially method, usage the fact that this is a conditional and also alters the sample area. The graph illustprices the new sample space. You already know the baby smiled more than eight seconds.Write a new f(x):displaystylef(x)=frac123-8=frac115\for 8
For the second way, usage the conditional formula (presented below) through the original distribution X ~ U (0, 23):For this trouble, A is (x > 12) and B is (x > 8).
Try It

A distribution is offered as X ~ U (0, 20). What is P(2

Example 3

The amount of time, in minutes, that a perchild should wait for a bus is uniformly distributed between zero and 15 minutes, inclusive.

What is the probability that a perboy waits fewer than 12.5 minutes?On the average, exactly how lengthy should a perkid wait? Find the suppose, μ, and also the traditional deviation, σ.Ninety percent of the time, the time a perboy have to wait falls below what value? This asks for the 90th percentile.


Let X = the variety of minutes a perkid have to wait for a bus. a = 0 and b = 15. X~ U(0, 15). Write the probability thickness attribute. displaystylef(x)=frac115-0=frac115\ for 0 ≤x ≤ 15.Find P (x displaystyleP(x x > 7) = 0.875

Example 5

Ace Heating and also Air Conditioning Service finds that the amount of time a repairmale demands to solve a heating system is uniformly dispersed in between 1.5 and also four hours. Let x = the moment needed to solve a heater. Then x ~ U (1.5, 4).

Find the probcapability that a randomly schosen heater repair calls for more than two hours.Find the probcapacity that a randomly schosen heater repair requires much less than three hours.Find the 30th percentile of furnace repair times.The longest 25% of heating system repair times take at leastern just how long? (In other words: find the minimum time for the longest 25% of repair times.) What percentile does this represent?Find the mean and also traditional deviation


To find f(x): displaystylef(x)=frac14-1.5=12.5 ext so f(x)=0.4\P(x > 2) = (base)(height) = (4 – 2)(0.4) = 0.8
Unicreate Distribution between 1.5 and 4 via shaded location in between 2 and also 4 representing the probcapability that the repair timex is better than twoP(x x = 1.5 and also x = 3. Keep in mind that the shaded location starts at x = 1.5 quite than at x = 0; considering that X ~ U (1.5, 4), x can not be much less than 1.5.
Unidevelop Distribution in between 1.5 and also four through shaded area between 1.5 and also 3 representing the probability that the repair time x is much less than three
P (x P(x k = 2.25 , derived by including 1.5 to both sidesThe 30th percentile of repair times is 2.25 hours. 30% of repair times are 2.5 hrs or much less.
Uniform Distribution between 1.5 and 4 with a space of 0.25 shaded to the ideal representing the longest 25% of repair times.P(x > k) = 0.25P(x > k) = (base)(height) = (4 – k)(0.4)0.25 = (4 – k)(0.4); Solve for k:0.625 = 4 − k, obtained by separating both sides by 0.4

−3.375 = − k, derived by subtracting four from both sides: k = 3.375

The longest 25% of heater repairs take at least 3.375 hours (3.375 hrs or longer).

Note: Due to the fact that 25% of repair times are 3.375 hours or much longer, that means that 75% of repair times are 3.375 hours or less. 3.375 hrs is the 75th percentile of heater repair ext and sigma=sqrtfrac(b-a)^212\mu=frac1.5+42=2.75 ext hours and also sigma=sqrtfrac(4-1.5)^212= 0.7217 ext hours\Try ItThe amount of time a service technician needs to change the oil in a auto is uniformly dispersed in between 11 and 21 minutes. Let X = the moment necessary to adjust the oil on a vehicle.

Write the random variable X in words. X = __________________.Write the circulation.Graph the circulation.Find P (x > 19).Find the 50th percentile.Let X = the moment required to readjust the oil in a auto.X ~ U (11, 21).P (x > 19) = 0.2the 50th percentile is 16 minutes.

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McDougall, John A. The McDougall Program for Maximum Weight Loss. Plume, 1995.

Concept Review

If X has actually a unidevelop distribution wright here a x)=(b-x)(frac1b-a)\

Area Between c and d: displaystyleP{(c{pdf: displaystylef(x)=frac1b-a\ for a ≤ x ≤ bcdf: P(Xx) = displaystylefracx-ab-a\mean: displaystylemu=fraca+b2\traditional deviation: displaystylesigma=sqrtfrac(b-a)^212\P(c