Regression Ne Demek
Kip Veilleux
In statistical modeling, regression analysis is a set of statistical processes for estimating the relationships between a dependent variable (often called the 'outcome' or 'response' variable, or a 'label' in machine learning parlance) and one or more independent variables (often called 'predictors', 'covariates', 'explanatory variables' or 'features'). The most common form of regression analysis is linear regression, in which one finds the line (or a more complex linear combination) that most closely fits the data according to a specific mathematical criterion. For example, the method of ordinary least squares computes the unique line (or hyperplane) that minimizes the sum of squared differences between the true data and that line (or hyperplane). For specific mathematical reasons (see linear regression), this allows the researcher to estimate the conditional expectation (or population average value) of the dependent variable when the independent variables take on a given set of values. Less common forms of regression use slightly different procedures to estimate alternative location parameters (e.g., quantile regression or Necessary Condition Analysis[1]) or estimate the conditional expectation across a broader collection of non-linear models (e.g., nonparametric regression).
Regression analysis is primarily used for two conceptually distinct purposes. First, regression analysis is widely used for prediction and forecasting, where its use has substantial overlap with the field of machine learning. Second, in some situations regression analysis can be used to infer causal relationships between the independent and dependent variables. Importantly, regressions by themselves only reveal relationships between a dependent variable and a collection of independent variables in a fixed dataset. To use regressions for prediction or to infer causal relationships, respectively, a researcher must carefully justify why existing relationships have predictive power for a new context or why a relationship between two variables has a causal interpretation. The latter is especially important when researchers hope to estimate causal relationships using observational data.[2][3]
The term "regression" was coined by Francis Galton in the 19th century to describe a biological phenomenon. The phenomenon was that the heights of descendants of tall ancestors tend to regress down towards a normal average (a phenomenon also known as regression toward the mean).[7][8]For Galton, regression had only this biological meaning,[9][10] but his work was later extended by Udny Yule and Karl Pearson to a more general statistical context.[11][12] In the work of Yule and Pearson, the joint distribution of the response and explanatory variables is assumed to be Gaussian. This assumption was weakened by R.A. Fisher in his works of 1922 and 1925.[13][14][15] Fisher assumed that the conditional distribution of the response variable is Gaussian, but the joint distribution need not be. In this respect, Fisher's assumption is closer to Gauss's formulation of 1821.
Regression methods continue to be an area of active research. In recent decades, new methods have been developed for robust regression, regression involving correlated responses such as time series and growth curves, regression in which the predictor (independent variable) or response variables are curves, images, graphs, or other complex data objects, regression methods accommodating various types of missing data, nonparametric regression, Bayesian methods for regression, regression in which the predictor variables are measured with error, regression with more predictor variables than observations, and causal inference with regression.
By itself, a regression is simply a calculation using the data. In order to interpret the output of regression as a meaningful statistical quantity that measures real-world relationships, researchers often rely on a number of classical assumptions. These assumptions often include:
Once a regression model has been constructed, it may be important to confirm the goodness of fit of the model and the statistical significance of the estimated parameters. Commonly used checks of goodness of fit include the R-squared, analyses of the pattern of residuals and hypothesis testing. Statistical significance can be checked by an F-test of the overall fit, followed by t-tests of individual parameters.
The response variable may be non-continuous ("limited" to lie on some subset of the real line). For binary (zero or one) variables, if analysis proceeds with least-squares linear regression, the model is called the linear probability model. Nonlinear models for binary dependent variables include the probit and logit model. The multivariate probit model is a standard method of estimating a joint relationship between several binary dependent variables and some independent variables. For categorical variables with more than two values there is the multinomial logit. For ordinal variables with more than two values, there are the ordered logit and ordered probit models. Censored regression models may be used when the dependent variable is only sometimes observed, and Heckman correction type models may be used when the sample is not randomly selected from the population of interest. An alternative to such procedures is linear regression based on polychoric correlation (or polyserial correlations) between the categorical variables. Such procedures differ in the assumptions made about the distribution of the variables in the population. If the variable is positive with low values and represents the repetition of the occurrence of an event, then count models like the Poisson regression or the negative binomial model may be used.
Regression models predict a value of the Y variable given known values of the X variables. Prediction within the range of values in the dataset used for model-fitting is known informally as interpolation. Prediction outside this range of the data is known as extrapolation. Performing extrapolation relies strongly on the regression assumptions. The further the extrapolation goes outside the data, the more room there is for the model to fail due to differences between the assumptions and the sample data or the true values.
There are no generally agreed methods for relating the number of observations versus the number of independent variables in the model. One method conjectured by Good and Hardin is N = m n \displaystyle N=m^n , where N \displaystyle N is the sample size, n \displaystyle n is the number of independent variables and m \displaystyle m is the number of observations needed to reach the desired precision if the model had only one independent variable.[22] For example, a researcher is building a linear regression model using a dataset that contains 1000 patients ( N \displaystyle N ). If the researcher decides that five observations are needed to precisely define a straight line ( m \displaystyle m ), then the maximum number of independent variables the model can support is 4, because
All major statistical software packages perform least squares regression analysis and inference. Simple linear regression and multiple regression using least squares can be done in some spreadsheet applications and on some calculators. While many statistical software packages can perform various types of nonparametric and robust regression, these methods are less standardized. Different software packages implement different methods, and a method with a given name may be implemented differently in different packages. Specialized regression software has been developed for use in fields such as survey analysis and neuroimaging.
As the name implies, multivariate regression is a technique that estimates a single regression model with more than one outcome variable. When there is more than one predictor variable in a multivariate regression model, the model is a multivariate multiple regression.
If you ran a separate OLS regression for each outcome variable, you would get exactly the same coefficients, standard errors, t- and p-values, and confidence intervals as shown above. So why conduct a multivariate regression? As we mentioned earlier, one of the advantages of using mvreg is that you can conduct tests of the coefficients across the different outcome variables. (Please note that many of these tests can be preformed after the manova command, although the process can be more difficult because a series of contrasts needs to be created.) In the examples below, we test four different hypotheses.
Sleep regression refers to a period of change in sleeping patterns experienced by babies. This change often occurs when infants who previously had no issues with sleep, settling well, or sleeping for prolonged periods suddenly experience wakeful nights and difficulty napping.
For instance, newborn babies only experience two sleep cycles at night, whereas adults experience 4, transitioning to the mature 4 phase cycle is thought to underlie sleep regression and disruptive sleep behavior.
Sleep regression varies between infants, but it typically occurs around 4, 6, 12, 18 and 24 months of age. Despite the lack of formal research on sleep regression and age-related stages, some older studies suggest these periods coincide with developmental milestones. For instance, the first regression, commonly at 4 months, is the beginning of an inevitable change in the sleeping pattern and often occurs in conjunction with teething, growing pain and beginning to move and roll. 8-month regressions coincide with learning to crawl and stand, separation anxiety can also be experienced during this period which may contribute to sleep disruption.
Other factors may also contribute to the onset of the biological shifts fundamental to sleep regression including significant growth spurts, disruptions in routine such as beginning nursery or illness.
To understand why sleep regression occurs it is important to understand the adult sleep cycle. Sleep consists of two phases: rapid eye movement (REM) and non-rapid eye movement (NREM) which is further divided into N1,2 and 3 in adults. Cycles in adults begin with NREM 1, the lightest phase from which people are easily awoken, followed by NREM 2, a deeper sleep during which heart rate and body temperature decrease. NREM 3 is the final NREM phase and is considered the deepest sleep stage and the most difficult stage to wake from. NREM 3 is known as slow-wave sleep and during this phase the body repairs tissue, muscle, and bone in addition to strengthening the immune system. REM is the final phase of sleep, and although this is associated with dreaming it is not considered a restful state. The EEG during this phase is similar to signals from an awake individual, the breathing rate is irregular and oxygen use of the brain increases due to high activity during this phase.
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