Standard Errors

# Standard errors #

One of the primary goals of data analysis is to estimate a characteristic of a population using a sample of data taken from that population. For example, we may wish to estimate the expected value (or population mean) using the sample mean. Or, we may wish to estimate the population value of the 0.75 quantile using the 0.75 quantile of the data.

An estimate of a population parameter is almost never exact. Thus, all estimates can be considered to have “error”. The error of an estimate is the value of the estimate minus the value being estimated (the value being estimated is often called the estimand, the “population parameter”, or the “target” of the estimate). In a very generic setting, the estimate may be denoted $$\hat{\theta}$$ (read “theta hat”) with the estimand denoted $$\theta$$ . The “hat” notation denotes that something is an estimate.

It is important to distinguish the error of an estimate from the “errors” in the data. For example, suppose we are studying the lung capacity of smokers. We may use a quantity called “Forced Expiratory Volume” (FEV) to quantify this trait. Suppose a person’s true FEV is 83, but we measure their FEV as 86. Then there is an error in the measured FEV value of +3 units. If we measure the FEV for 100 smokers and obtain a mean FEV for the sample of 79 units, and if the population mean FEV for all smokers is 80, then the estimation error for the population mean is -1 unit.

We can almost never know the estimation error exactly. If we knew the estimation error, we could subtract it from our estimate to recover the population value without any uncertainty. In the example above, if we knew that the estimation error for the mean FEV of smokers was -1 unit, with an estimated value of 79 units, then we could recover the population mean without uncertainty by subtracting the error from the estimate, 79 - (-1) = 80.

Since we can’t actually know the estimation error, we have to settle for something less specific than this. The standard error is, roughly speaking, the typical magnitude of the estimation error. By “typical”, we mean the magnitude of the estimation error that we would generally observe in a study like ours. The standard error is not the actual estimation error in our study, but instead tells us for studies using the methods that we are using and with a similar population as we are studying, what would the magnitude of the estimation error typically be. It is tempting to refer to the standard error as the “average” or “expected” estimation error, but this is not quite right, for reasons discussed further below.

In the above example of estimating the FEV of smokers, the standard error might be, say 1.5. That is, on average for the sample size and population under consideration, the estimated mean FEV tends to be off by around 1.5 units in one direction or the other. The specific estimation error of -1 units is not knowable, but it is not surprising that if the typical error has magnitude 1.5 units, then the error in one specific instance could be -1 unit.

In many data analysis settings, it is sufficient to report the estimate and its standard error. This is often written in the form 79 (1.5). That is, we write the estimate first, then the standard error next to it in parentheses. This tells the reader that the data supports a FEV value of 79 units, but this value has an error that can be either positive or negative, and tends to have magnitude of around 1.5 units.

We should point out that while the standard error is often described as the “average magnitude of the estimation error”, this is not quite correct. More exactly, the standard error is the square root of the average squared magnitude of the estimation error. The average of the square root of a random quantity X is not the same as the square root of the average of X.

## Standard error of the mean #

The most commonly encountered standard error arises in the setting where the population parameter of interest is the expected value, and the statistic we are using for estimation is the sample mean. If we have an independent and identically distributed sample, then the standard error in this setting is equal to the standard deviation of the measurements (often denoted $$\sigma$$ ) divided by the square root of the sample size (often denoted n). Thus the standard error of the mean (for IID data), often denoted SEM, is $$\sigma/\sqrt{n}$$ .

The expression for the standard error of the mean, $$\sigma/\sqrt{n}$$ , tells a great deal about what factors of the population and sample influence the precision of our estimate. To have a precise estimate, we want the standard error to be small. Thus, we see that we have a small standard error when either $$\sigma$$ is small, $$n$$ is large, or both. It is natural that having more data (a larger value of $$n$$ ) leads to a more precise estimate of the population mean. The fact that the sample size enters via its square root implies that we need to increase the sample size by a factor of four to reduce the standard error by a factor of two. Thus, small increases in the sample size do not bring a substantial improvement in precision. The value of $$\sigma$$ describes the variation in the individual values that we are studying. It should not be surprising that if these values are more dispersed, the estimated value will be less precise. We usually do not have control over the value of $$\sigma$$ , but it is important to understand how its value influences the uncertainty in our estimates of the population parameters of interest.

## Estimated standard errors and nuisance parameters #

The exact value of the SEM depends on the standard deviation of the measurements, $$\sigma$$ , but this value is almost never known. We can estimate it using the sample standard deviation, $$\hat{\sigma}$$ of the data values, and use the estimated standard deviation to produce an estimated standard error $$\hat{\sigma}/\sqrt{n}$$ . But this introduces yet more uncertainty into the analysis. A quantity like $$\sigma$$ that is not of direct interest, but that is needed to conduct uncertainty analysis on our quantity of direct interest (i.e. the mean), is called a nuisance parameter.

Nuisance parameters can complicate statistical analyses. The most common way of dealing with a nuisance parameter is to estimate it, and “plug-in” the estimated nuisance parameter to any formula that we are working with. This is sometimes called a “plug-in approach”.

## Standard errors for differences #

Differences, or contrasts arise very frequently in data analysis. For example, suppose we are interested in comparing the FEV values between smokers and non-smokers. A natural way to do this would be to obtain samples of $$n_{\rm s}$$ smokers and $$n_{\rm ns}$$ non-smokers. Then we calculate the sample means $$\overline{\rm FEV}_{\rm s}$$ and $$\overline{\rm FEV}_{\rm ns}$$ for each group, and then calculate their difference $$\overline{\rm FEV}_{\rm s} - \overline{\rm FEV}_{\rm ns}$$ . This is an estimate of the population contrast $$E[{\rm FEV}_{\rm s}] - E[{\rm FEV}_{\rm ns}]$$ .

Like any estimate, $$\overline{\rm FEV}_{\rm s} - \overline{\rm FEV}_{\rm ns}$$ is not equal to the corresponding estimand $$E[{\rm FEV}_{\rm s}] - E[{\rm FEV}_{\rm ns}]$$ . A standard error can be used to quantify the typical error in this setting. The standard error for $$\overline{\rm FEV}_{\rm s}$$ alone is $$\sigma_{\rm s}/\sqrt{\rm n_s}$$ , where $$\sigma_s$$ is the standard deviation of FEV values for smokers. Similarly, the standard error for $$\overline{\rm FEV}_{\rm ns}$$ alone is $$\sigma_{\rm ns}/\sqrt{n_{\rm ns}}$$ , where $$\sigma_{\rm s}$$ is the standard deviation of FEV values for non-smokers.

A very general result from probability tells us how to combine the uncertainty in two independent estimates when we subtract them. If $$s_1$$ and $$s_2$$ are two standard deviations, say for independent estimates $$A$$ and $$B$$ , then the standard deviation for $$A-B$$ is $$\sqrt{s_1^2+s_2^2}$$ . Thus, the standard error for $$\overline{\rm FEV}_{\rm s} - \overline{\rm FEV}_{\rm ns}$$ is

$$\sqrt{\sigma_{\rm s}^2/n_{\rm s} + \sigma_{\rm ns}^2/n_{\rm ns}}.$$

Note that this standard error contains two nuisance parameters, $$\sigma_{\rm s}$$ and $$\sigma_{\rm ns}$$ . We can get an estimated standard error by plugging their estimated values into the exact standard error:

$$\sqrt{\hat{\sigma}_{\rm s}^2/n_{\rm s} + \hat{\sigma}_{\rm ns}^2/n_{\rm ns}}.$$

## Standard errors for proportions #

A proportion is actually a mean, but proportions arise so often that their standard error is usually calculated in a special way. The proportion is the number of times that something happens, out of a given set of opportunities for it to happen. Suppose we ask four people whether they have consumed alcohol in the past week, and the responses are “yes”, “no”, “no”, and “yes”. The proportion of people responding “yes” is 1/2. If we code “yes” as 1 and “no” as zero, then the data can be represented as 1, 0, 0, 1, and the proportion of “yes” responses is (1 + 0 + 0 + 1) / 4 = 1/2. This example illustrates why proportions are actually equivalent to means.

Suppose that $$X$$ is a random variable representing a binary outcome, say $$X=1$$ corresponds to a respondent stating that they drank alcohol in the last week, and $$X=0$$ corresponds to the respondent stating that they have not had any alcohol in the past week. The “event probability” is denoted $$p$$ and defined to be $$P(X=1)$$ . A fact that we will not prove here is that the variance of $$X$$ is $$p(1-p)$$ . Therefore the standard error for the sample proportion based on $$n$$ independent and identically distributed observations of $$X$$ is $$\sqrt{p(1-p)/n}$$ .

## Standard errors for other statistics #

Every statistic has a standard error, but in many cases the exact form of the standard error is difficult to derive. More advanced statistics courses develop methods for calculating and approximating standard errors for more difficult settings than we consider here. We will provide one more standard error here because it is quite useful and extremely simple. If we calculate the sample correlation coefficient for paired values $$(x_1, y_1), \ldots, (x_n, y_n)$$ , an iid sample from a bivariate distribution $$(X, Y)$$ , then the approximate standard deviation (or standard error) for the sample correlation coefficient is simply $$1/\sqrt{n}$$ . This is an approximate standard error, but works well in many settings.