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This lesson will explain onetailed and twotailed tests.
This is another example of a onetailed test. The null hypothesis says that the average amount of cola in the bottle is 1 liter over all the bottles that Liter O'Cola makes. The alternative is that maybe we think that it's less than 1. They're underfilling the bottles. The average amount is less than 1 liter.
Onetailed tests have two versions, a lefttailed test or a righttailed test. A left tailed test is where you say in the alternative hypothesis that it's less than this claimed parameter. A right tailed test means that the alternative hypothesis is larger than the claimed parameter.
One and twotailed tests  Wikipedia
The isn’t used very often (because we rarely know the actual population ). However, it’s a good idea to understand how it works as it’s one of the simplest tests you can perform in hypothesis testing. In English class you got to learn the basics (like grammar and spelling) before you could write a story; think of one sample z tests as the foundation for understanding more complex hypothesis testing. This page contains two hypothesis testing examples for .
The main purpose of statistics is to test a hypothesis. For example, you might run an experiment and find that a certain drug is effective at treating headaches. But if you can’t repeat that experiment, no one will take your results seriously. A good example of this was the discovery, which petered into obscurity because no one was able to duplicate the results.
Onetailed and twotailed tests  The Analysis Factor
The right tailed test and the left tailed test are examples of onetailed tests. They are called “one tailed” tests because the (the area where you would ) is only in one tail. The two tailed test is called a two tailed test because the rejection region can be in either tail.
On the other hand, it would be inappropriate (and perhaps, unethical) to run a onetailed test for this scenario in the opposite direction (i.e. to show the drug is more effective). This sounds reasonable until you consider there may be certain circumstances where the drug is less effective. If you fail to test for that, your research will be useless.
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I was recently asked about when to use one and two tailed tests
For example, you develop a drug which you think is just as effective as a drug already on the market (it also happens to be cheaper). You could run a twotailed test (to test that it is more effective and to also check that it is less effective). But you don’t really care about it being more effective, just that it isn’t any less effective (after all, your drug is cheaper). You can run a onetailed test to check that your drug is at least as effective as the existing drug.
Answer to Hypothesis Test  onetail or twotail a
Consider both directions when deciding if you should run a one tailed test or two. If you can skip one tail and it’s not irresponsible or unethical to do so, then you can run a onetailed test.
st: Re: onetailed tests  Data Analysis and Statistical …
A right tailed test (sometimes called an upper test) is where your hypothesis statement contains a greater than (>) symbol. In other words, the inequality points to the right. For example, you might be comparing the life of batteries before and after a manufacturing change. If you want to know if the battery life is greater than the original (let’s say 90 hours), your hypothesis statements might be:
: No change (H_{0} = 90).
: Battery life has increased (H_{1}) > 90.
When should we use onetailed hypothesis testing?  …
In this tutorial, you're going to learn about the difference between a onetailed and a twotailed test in a hypothesis test. You will specifically focus on:
Although onetailed hypothesis tests are commonly used, ..
A highend computer manufacturer sets the retail cost of their computers based in the manufacturing cost, which is $1800. However, the company thinks there are hidden costs and that the cost to manufacture the computers is actually much more. The company randomly selects 40 computers from its facilities and finds that the mean cost to produce a computer is $1950 with a standard deviation of $500. Run a hypothesis test to see if this thought is true.
Should I use Onetailed or Twotailed Hypothesis Tests?
Here, the null hypothesis is that the mean grams of sugar will be the same as it was before, 35. What about the alternative hypothesis? Well, if they've changed their formula, you don't know if they added more sugar or put in less sugar. But they're only going to be in trouble if they put in a different amount of sugar than before. The mean, a number of grams of sugar in the bottle, is different than 35. And this is a twotailed test. They're going to be in trouble if they put in significantly more than 35 or significantly less.
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