Nicholas Cox is a statistically minded geographer at Durham University. He contributes talks, postings, FAQ, and programs to the Stata user community. He has also coauthored 16 commands in official Stata. He was an author of several inserts in the Stata Technical Bulletin and is Editor-at-Large of the Stata Journal.

• Introducing Stata tips Stata tip 1: The eform() option of regress, R. Newson Stata tip 2: Building with floors and ceilings, N. J. Cox Stata tip 3: How to be assertive, W. Gould Stata tip 4: Using display as an online calculator, P. Ryan Stata tip 5: Ensuring programs preserve dataset order, R. Newson Stata tip 6: Inserting awkward characters in the plot, N. J. Cox Stata tip 7: Copying and pasting under Windows, S. Driver and P. Royston Stata tip 8: Splitting time-span records with categorical time-varying covariates, B. Jann Stata tip 9: Following special sequences, N. J. Cox Stata tip 10: Fine control of axis title positions, P. Ryan and N. Winter Stata tip 11: The nolog option with maximum-likelihood modeling commands, P. Royston Stata tip 12: Tuning the plot region aspect ratio, N. J. Cox Stata tip 13: generate and replace use the current sort order, R. Newson Stata tip 14: Using value labels in expressions, K. Higbee Stata tip 15: Function graphs on the fly, N. J. Cox Stata tip 16: Using input to generate variables, U. Kohler Stata tip 17: Filling in the gaps, N. J. Cox Stata tip 18: Making keys functional, S. Driver Stata tip 19: A way to leaner, faster graphs, P. Royston Stata tip 20: Generating histogram bin variables, D. A. Harrison Stata tip 21: The arrows of outrageous fortune, N. J. Cox Stata tip 22: Variable name abbreviation, P. Ryan Stata tip 23: Regaining control over axis ranges, N. Winter Stata tip 24: Axis labels on two or more levels, N. J. Cox Stata tip 25: Sequence index plots, U. Kohler and C. Brzinsky-Fay Stata tip 26: Maximizing compatibility between Macintosh and Windows, M. S. Hanson Stata tip 27: Classifying data points on scatter plots, N. J. Cox Stata tip 28: Precise control of dataset sort order, P. Schumm Stata tip 29: For all times and all places, C. H. Franklin Stata tip 30: May the source be with you, N. J. Cox Stata tip 31: Scalar or variable? The problem of ambiguous names, G. I. Kolev Stata tip 32: Do not stop, S. P. Jenkins Stata tip 33: Sweet sixteen: Hexadecimal formats and precision problems, N. J. Cox Stata tip 34: Tabulation by listing, D. A. Harrison Stata tip 35: Detecting whether data have changed, W. Gould Stata tip 36: Which observations?, N. J. Cox Stata tip 37: And the last shall be first, C. F. Baum Stata tip 38: Testing for groupwise heteroskedasticity, C. F. Baum Stata tip 39: In a list or out? In a range or out?, N. J. Cox Stata tip 40: Taking care of business, C. F. Baum Stata tip 41: Monitoring loop iterations, D. A. Harrison Stata tip 42: The overlay problem: Offset for clarity, J. Cui Stata tip 43: Remainders, selections, sequences, extractions: Uses of the modulus, N. J. Cox Stata tip 44: Get a handle on your sample, B. Jann Stata tip 45: Getting those data into shape, C. F. Baum and N. J. Cox Stata tip 46: Step we gaily, on we go, R. Williams Stata tip 47: Quantile–quantile plots without programming, N. J. Cox Stata tip 48: Discrete uses for uniform(), M. L. Buis Stata tip 49: Range frame plots, S. Merryman Stata tip 50: Efficient use of summarize, N. J. Cox Stata tip 51: Events in intervals, N. J. Cox Stata tip 52: Generating composite categorical variables, N. J. Cox Stata tip 53: Where did my p-values go?, M. L. Buis Stata tip 54: Post your results, P. Van Kerm Stata tip 55: Better axis labeling for time points and time intervals, N. J. Cox Stata tip 56: Writing parameterized text files, R. Gini Stata tip 57: How to reinstall Stata, W. Gould Stata tip 58: nl is not just for nonlinear models, B. P. Poi Stata tip 59: Plotting on any transformed scale, N. J. Cox Stata tip 60: Making fast and easy changes to files with filefilter, A. R. Riley Stata tip 61: Decimal commas in results output and data input, N. J. Cox Stata tip 62: Plotting on reversed scales, N. J. Cox and N. L. M. Barlow Stata tip 63: Modeling proportions, C. F. Baum Stata tip 64: Cleaning up user-entered string variables, J. Herrin and E. Poen Stata tip 65: Beware the backstabbing backslash, N. J. Cox Stata tip 66: ds—A hidden gem, M. Weiss Stata tip 67: J() now has greater replicating powers, N. J. Cox Stata tip 68: Week assumptions, N. J. Cox Stata tip 69: Producing log files based on successful interactive commands, A. R. Riley Stata tip 70: Beware the evaluating equal sign, N. J. Cox Stata tip 71: The problem of split identity, or how to group dyads, N. J. Cox Stata tip 72: Using the Graph Recorder to create a pseudograph scheme, K. Crow Stata tip 73: append with care!, C. F. Baum Stata tip 74: firstonly, a new option for tab2, R. G. Gutierrez and P. A. Lachenbruch Stata tip 75: Setting up Stata for a presentation, K. Crow Stata tip 76: Separating seasonal time series, N. J. Cox Stata tip 77: (Re)using macros in multiple do-files, J. Herrin Stata tip 78: Going gray gracefully: Highlighting subsets and downplaying substrates, N. J. Cox Stata tip 79: Optional arguments to options, N. J. Cox Stata tip 80: Constructing a group variable with specified group sizes, M. Weiss Stata tip 81: A table of graphs, M. L. Buis and M. Weiss Stata tip 82: Grounds for grids on graphs, N. J. Cox Stata tip 83: Merging multilingual datasets, D. L. Golbe Stata tip 84: Summing missings, N. J. Cox Stata tip 85: Looping over nonintegers, N. J. Cox Stata tip 86: The missing() function, B. Rising Stata tip 87: Interpretation of interactions in nonlinear models, M. L. Buis Stata tip 88: Efficiently evaluating elastics with the margins command, C. F. Baum Stata tip 89: Estimating means and percentiles following multiple imputation, P. A. Lachenbruch Stata tip 90: Displaying partial results, M. Weiss Stata tip 91: Putting unabbreviated varlists into local macros, N. J. Cox Stata tip 92: Manual implementation of permutations and bootstraps, L. Ãngquist Stata tip 93: Handling multiple y axes on twoway graphs, V. Wiggins Stata tip 94: Manipulation of prediction parameters for parametric survival regression models, T. Boswell and R. G. Gutierrez Stata tip 95: Estimation of error covariances in a linear model, N. J. Horton Stata tip 96: Cube roots, N. J. Cox Stata tip 97: Getting at ρ's and σ's, M. L. Buis Stata tip 98: Counting substrings within strings, N. J. Cox Stata tip 99: Taking extra care with encode, C. Schechter Stata tip 100: Mata and the case of the missing macros, W. Gould and N. J. Cox Stata tip 101: Previous but different, N. J. Cox Stata tip 102: Highlighting specific bars, N. J. Cox Stata tip 103: Expressing confidence with gradations, U. Kohler and S. Eckman Stata tip 104: Added text and title options, N. J. Cox Stata tip 105: Daily dates with missing days, S. J. Samuels and N. J. Cox Stata tip 106: With or without reference, M. L. Buis Stata tip 107: The baseline is now reported, M. L. Buis Stata tip 108: On adding and constraining, M. L. Buis Stata tip 109: How to combine variables with missing values, P. A. Lachenbruch Stata tip 110: How to get the optimal k-means cluster solution, A. Makles Stata tip 111: More on working with weeks, N. J. Cox Stata tip 112: Where did my p-values go? (Part 2), M. L. Buis Stata tip 113: Changing a variable's format: What it does and does not mean, N. J. Cox Stata tip 114: Expand paired dates to pairs of dates, N. J. Cox Stata tip 115: How to properly estimate the multinomial probit model with heteroskedastic errors, M. Herrmann Stata tip 116: Where did my p-values go? (Part 3), M. L. Buis Stata tip 117: graph combine—Combining graphs, L. Ãngquist Stata tip 118: Orthogonalizing powered and product terms using residual centering, C. Sauer Stata tip 119: Expanding datasets for graphical ends, N. J. Cox

Nicholas Cox is a statistically minded geographer at Durham University. He contributes talks, postings, FAQ, and programs to the Stata user community. He has also coauthored 16 commands in official Stata. He was an author of several inserts in the Stata Technical Bulletin and is Editor-at-Large of the Stata Journal.

• Introducing Stata tips Stata tip 1: The eform() option of regress, R. Newson Stata tip 2: Building with floors and ceilings, N. J. Cox Stata tip 3: How to be assertive, W. Gould Stata tip 4: Using display as an online calculator, P. Ryan Stata tip 5: Ensuring programs preserve dataset order, R. Newson Stata tip 6: Inserting awkward characters in the plot, N. J. Cox Stata tip 7: Copying and pasting under Windows, S. Driver and P. Royston Stata tip 8: Splitting time-span records with categorical time-varying covariates, B. Jann Stata tip 9: Following special sequences, N. J. Cox Stata tip 10: Fine control of axis title positions, P. Ryan and N. Winter Stata tip 11: The nolog option with maximum-likelihood modeling commands, P. Royston Stata tip 12: Tuning the plot region aspect ratio, N. J. Cox Stata tip 13: generate and replace use the current sort order, R. Newson Stata tip 14: Using value labels in expressions, K. Higbee Stata tip 15: Function graphs on the fly, N. J. Cox Stata tip 16: Using input to generate variables, U. Kohler Stata tip 17: Filling in the gaps, N. J. Cox Stata tip 18: Making keys functional, S. Driver Stata tip 19: A way to leaner, faster graphs, P. Royston Stata tip 20: Generating histogram bin variables, D. A. Harrison Stata tip 21: The arrows of outrageous fortune, N. J. Cox Stata tip 22: Variable name abbreviation, P. Ryan Stata tip 23: Regaining control over axis ranges, N. Winter Stata tip 24: Axis labels on two or more levels, N. J. Cox Stata tip 25: Sequence index plots, U. Kohler and C. Brzinsky-Fay Stata tip 26: Maximizing compatibility between Macintosh and Windows, M. S. Hanson Stata tip 27: Classifying data points on scatter plots, N. J. Cox Stata tip 28: Precise control of dataset sort order, P. Schumm Stata tip 29: For all times and all places, C. H. Franklin Stata tip 30: May the source be with you, N. J. Cox Stata tip 31: Scalar or variable? The problem of ambiguous names, G. I. Kolev Stata tip 32: Do not stop, S. P. Jenkins Stata tip 33: Sweet sixteen: Hexadecimal formats and precision problems, N. J. Cox Stata tip 34: Tabulation by listing, D. A. Harrison Stata tip 35: Detecting whether data have changed, W. Gould Stata tip 36: Which observations?, N. J. Cox Stata tip 37: And the last shall be first, C. F. Baum Stata tip 38: Testing for groupwise heteroskedasticity, C. F. Baum Stata tip 39: In a list or out? In a range or out?, N. J. Cox Stata tip 40: Taking care of business, C. F. Baum Stata tip 41: Monitoring loop iterations, D. A. Harrison Stata tip 42: The overlay problem: Offset for clarity, J. Cui Stata tip 43: Remainders, selections, sequences, extractions: Uses of the modulus, N. J. Cox Stata tip 44: Get a handle on your sample, B. Jann Stata tip 45: Getting those data into shape, C. F. Baum and N. J. Cox Stata tip 46: Step we gaily, on we go, R. Williams Stata tip 47: Quantile–quantile plots without programming, N. J. Cox Stata tip 48: Discrete uses for uniform(), M. L. Buis Stata tip 49: Range frame plots, S. Merryman Stata tip 50: Efficient use of summarize, N. J. Cox Stata tip 51: Events in intervals, N. J. Cox Stata tip 52: Generating composite categorical variables, N. J. Cox Stata tip 53: Where did my p-values go?, M. L. Buis Stata tip 54: Post your results, P. Van Kerm Stata tip 55: Better axis labeling for time points and time intervals, N. J. Cox Stata tip 56: Writing parameterized text files, R. Gini Stata tip 57: How to reinstall Stata, W. Gould Stata tip 58: nl is not just for nonlinear models, B. P. Poi Stata tip 59: Plotting on any transformed scale, N. J. Cox Stata tip 60: Making fast and easy changes to files with filefilter, A. R. Riley Stata tip 61: Decimal commas in results output and data input, N. J. Cox Stata tip 62: Plotting on reversed scales, N. J. Cox and N. L. M. Barlow Stata tip 63: Modeling proportions, C. F. Baum Stata tip 64: Cleaning up user-entered string variables, J. Herrin and E. Poen Stata tip 65: Beware the backstabbing backslash, N. J. Cox Stata tip 66: ds—A hidden gem, M. Weiss Stata tip 67: J() now has greater replicating powers, N. J. Cox Stata tip 68: Week assumptions, N. J. Cox Stata tip 69: Producing log files based on successful interactive commands, A. R. Riley Stata tip 70: Beware the evaluating equal sign, N. J. Cox Stata tip 71: The problem of split identity, or how to group dyads, N. J. Cox Stata tip 72: Using the Graph Recorder to create a pseudograph scheme, K. Crow Stata tip 73: append with care!, C. F. Baum Stata tip 74: firstonly, a new option for tab2, R. G. Gutierrez and P. A. Lachenbruch Stata tip 75: Setting up Stata for a presentation, K. Crow Stata tip 76: Separating seasonal time series, N. J. Cox Stata tip 77: (Re)using macros in multiple do-files, J. Herrin Stata tip 78: Going gray gracefully: Highlighting subsets and downplaying substrates, N. J. Cox Stata tip 79: Optional arguments to options, N. J. Cox Stata tip 80: Constructing a group variable with specified group sizes, M. Weiss Stata tip 81: A table of graphs, M. L. Buis and M. Weiss Stata tip 82: Grounds for grids on graphs, N. J. Cox Stata tip 83: Merging multilingual datasets, D. L. Golbe Stata tip 84: Summing missings, N. J. Cox Stata tip 85: Looping over nonintegers, N. J. Cox Stata tip 86: The missing() function, B. Rising Stata tip 87: Interpretation of interactions in nonlinear models, M. L. Buis Stata tip 88: Efficiently evaluating elastics with the margins command, C. F. Baum Stata tip 89: Estimating means and percentiles following multiple imputation, P. A. Lachenbruch Stata tip 90: Displaying partial results, M. Weiss Stata tip 91: Putting unabbreviated varlists into local macros, N. J. Cox Stata tip 92: Manual implementation of permutations and bootstraps, L. Ãngquist Stata tip 93: Handling multiple y axes on twoway graphs, V. Wiggins Stata tip 94: Manipulation of prediction parameters for parametric survival regression models, T. Boswell and R. G. Gutierrez Stata tip 95: Estimation of error covariances in a linear model, N. J. Horton Stata tip 96: Cube roots, N. J. Cox Stata tip 97: Getting at ρ's and σ's, M. L. Buis Stata tip 98: Counting substrings within strings, N. J. Cox Stata tip 99: Taking extra care with encode, C. Schechter Stata tip 100: Mata and the case of the missing macros, W. Gould and N. J. Cox Stata tip 101: Previous but different, N. J. Cox Stata tip 102: Highlighting specific bars, N. J. Cox Stata tip 103: Expressing confidence with gradations, U. Kohler and S. Eckman Stata tip 104: Added text and title options, N. J. Cox Stata tip 105: Daily dates with missing days, S. J. Samuels and N. J. Cox Stata tip 106: With or without reference, M. L. Buis Stata tip 107: The baseline is now reported, M. L. Buis Stata tip 108: On adding and constraining, M. L. Buis Stata tip 109: How to combine variables with missing values, P. A. Lachenbruch Stata tip 110: How to get the optimal k-means cluster solution, A. Makles Stata tip 111: More on working with weeks, N. J. Cox Stata tip 112: Where did my p-values go? (Part 2), M. L. Buis Stata tip 113: Changing a variable's format: What it does and does not mean, N. J. Cox Stata tip 114: Expand paired dates to pairs of dates, N. J. Cox Stata tip 115: How to properly estimate the multinomial probit model with heteroskedastic errors, M. Herrmann Stata tip 116: Where did my p-values go? (Part 3), M. L. Buis Stata tip 117: graph combine—Combining graphs, L. Ãngquist Stata tip 118: Orthogonalizing powered and product terms using residual centering, C. Sauer Stata tip 119: Expanding datasets for graphical ends, N. J. Cox

Nicholas Cox is a statistically minded geographer at Durham University. He contributes talks, postings, FAQ, and programs to the Stata user community. He has also coauthored 16 commands in official Stata. He was an author of several inserts in the Stata Technical Bulletin and is Editor-at-Large of the Stata Journal.

• Introducing Stata tips Stata tip 1: The eform() option of regress, R. Newson Stata tip 2: Building with floors and ceilings, N. J. Cox Stata tip 3: How to be assertive, W. Gould Stata tip 4: Using display as an online calculator, P. Ryan Stata tip 5: Ensuring programs preserve dataset order, R. Newson Stata tip 6: Inserting awkward characters in the plot, N. J. Cox Stata tip 7: Copying and pasting under Windows, S. Driver and P. Royston Stata tip 8: Splitting time-span records with categorical time-varying covariates, B. Jann Stata tip 9: Following special sequences, N. J. Cox Stata tip 10: Fine control of axis title positions, P. Ryan and N. Winter Stata tip 11: The nolog option with maximum-likelihood modeling commands, P. Royston Stata tip 12: Tuning the plot region aspect ratio, N. J. Cox Stata tip 13: generate and replace use the current sort order, R. Newson Stata tip 14: Using value labels in expressions, K. Higbee Stata tip 15: Function graphs on the fly, N. J. Cox Stata tip 16: Using input to generate variables, U. Kohler Stata tip 17: Filling in the gaps, N. J. Cox Stata tip 18: Making keys functional, S. Driver Stata tip 19: A way to leaner, faster graphs, P. Royston Stata tip 20: Generating histogram bin variables, D. A. Harrison Stata tip 21: The arrows of outrageous fortune, N. J. Cox Stata tip 22: Variable name abbreviation, P. Ryan Stata tip 23: Regaining control over axis ranges, N. Winter Stata tip 24: Axis labels on two or more levels, N. J. Cox Stata tip 25: Sequence index plots, U. Kohler and C. Brzinsky-Fay Stata tip 26: Maximizing compatibility between Macintosh and Windows, M. S. Hanson Stata tip 27: Classifying data points on scatter plots, N. J. Cox Stata tip 28: Precise control of dataset sort order, P. Schumm Stata tip 29: For all times and all places, C. H. Franklin Stata tip 30: May the source be with you, N. J. Cox Stata tip 31: Scalar or variable? The problem of ambiguous names, G. I. Kolev Stata tip 32: Do not stop, S. P. Jenkins Stata tip 33: Sweet sixteen: Hexadecimal formats and precision problems, N. J. Cox Stata tip 34: Tabulation by listing, D. A. Harrison Stata tip 35: Detecting whether data have changed, W. Gould Stata tip 36: Which observations?, N. J. Cox Stata tip 37: And the last shall be first, C. F. Baum Stata tip 38: Testing for groupwise heteroskedasticity, C. F. Baum Stata tip 39: In a list or out? In a range or out?, N. J. Cox Stata tip 40: Taking care of business, C. F. Baum Stata tip 41: Monitoring loop iterations, D. A. Harrison Stata tip 42: The overlay problem: Offset for clarity, J. Cui Stata tip 43: Remainders, selections, sequences, extractions: Uses of the modulus, N. J. Cox Stata tip 44: Get a handle on your sample, B. Jann Stata tip 45: Getting those data into shape, C. F. Baum and N. J. Cox Stata tip 46: Step we gaily, on we go, R. Williams Stata tip 47: Quantile–quantile plots without programming, N. J. Cox Stata tip 48: Discrete uses for uniform(), M. L. Buis Stata tip 49: Range frame plots, S. Merryman Stata tip 50: Efficient use of summarize, N. J. Cox Stata tip 51: Events in intervals, N. J. Cox Stata tip 52: Generating composite categorical variables, N. J. Cox Stata tip 53: Where did my p-values go?, M. L. Buis Stata tip 54: Post your results, P. Van Kerm Stata tip 55: Better axis labeling for time points and time intervals, N. J. Cox Stata tip 56: Writing parameterized text files, R. Gini Stata tip 57: How to reinstall Stata, W. Gould Stata tip 58: nl is not just for nonlinear models, B. P. Poi Stata tip 59: Plotting on any transformed scale, N. J. Cox Stata tip 60: Making fast and easy changes to files with filefilter, A. R. Riley Stata tip 61: Decimal commas in results output and data input, N. J. Cox Stata tip 62: Plotting on reversed scales, N. J. Cox and N. L. M. Barlow Stata tip 63: Modeling proportions, C. F. Baum Stata tip 64: Cleaning up user-entered string variables, J. Herrin and E. Poen Stata tip 65: Beware the backstabbing backslash, N. J. Cox Stata tip 66: ds—A hidden gem, M. Weiss Stata tip 67: J() now has greater replicating powers, N. J. Cox Stata tip 68: Week assumptions, N. J. Cox Stata tip 69: Producing log files based on successful interactive commands, A. R. Riley Stata tip 70: Beware the evaluating equal sign, N. J. Cox Stata tip 71: The problem of split identity, or how to group dyads, N. J. Cox Stata tip 72: Using the Graph Recorder to create a pseudograph scheme, K. Crow Stata tip 73: append with care!, C. F. Baum Stata tip 74: firstonly, a new option for tab2, R. G. Gutierrez and P. A. Lachenbruch Stata tip 75: Setting up Stata for a presentation, K. Crow Stata tip 76: Separating seasonal time series, N. J. Cox Stata tip 77: (Re)using macros in multiple do-files, J. Herrin Stata tip 78: Going gray gracefully: Highlighting subsets and downplaying substrates, N. J. Cox Stata tip 79: Optional arguments to options, N. J. Cox Stata tip 80: Constructing a group variable with specified group sizes, M. Weiss Stata tip 81: A table of graphs, M. L. Buis and M. Weiss Stata tip 82: Grounds for grids on graphs, N. J. Cox Stata tip 83: Merging multilingual datasets, D. L. Golbe Stata tip 84: Summing missings, N. J. Cox Stata tip 85: Looping over nonintegers, N. J. Cox Stata tip 86: The missing() function, B. Rising Stata tip 87: Interpretation of interactions in nonlinear models, M. L. Buis Stata tip 88: Efficiently evaluating elastics with the margins command, C. F. Baum Stata tip 89: Estimating means and percentiles following multiple imputation, P. A. Lachenbruch Stata tip 90: Displaying partial results, M. Weiss Stata tip 91: Putting unabbreviated varlists into local macros, N. J. Cox Stata tip 92: Manual implementation of permutations and bootstraps, L. Ãngquist Stata tip 93: Handling multiple y axes on twoway graphs, V. Wiggins Stata tip 94: Manipulation of prediction parameters for parametric survival regression models, T. Boswell and R. G. Gutierrez Stata tip 95: Estimation of error covariances in a linear model, N. J. Horton Stata tip 96: Cube roots, N. J. Cox Stata tip 97: Getting at ρ's and σ's, M. L. Buis Stata tip 98: Counting substrings within strings, N. J. Cox Stata tip 99: Taking extra care with encode, C. Schechter Stata tip 100: Mata and the case of the missing macros, W. Gould and N. J. Cox Stata tip 101: Previous but different, N. J. Cox Stata tip 102: Highlighting specific bars, N. J. Cox Stata tip 103: Expressing confidence with gradations, U. Kohler and S. Eckman Stata tip 104: Added text and title options, N. J. Cox Stata tip 105: Daily dates with missing days, S. J. Samuels and N. J. Cox Stata tip 106: With or without reference, M. L. Buis Stata tip 107: The baseline is now reported, M. L. Buis Stata tip 108: On adding and constraining, M. L. Buis Stata tip 109: How to combine variables with missing values, P. A. Lachenbruch Stata tip 110: How to get the optimal k-means cluster solution, A. Makles Stata tip 111: More on working with weeks, N. J. Cox Stata tip 112: Where did my p-values go? (Part 2), M. L. Buis Stata tip 113: Changing a variable's format: What it does and does not mean, N. J. Cox Stata tip 114: Expand paired dates to pairs of dates, N. J. Cox Stata tip 115: How to properly estimate the multinomial probit model with heteroskedastic errors, M. Herrmann Stata tip 116: Where did my p-values go? (Part 3), M. L. Buis Stata tip 117: graph combine—Combining graphs, L. Ãngquist Stata tip 118: Orthogonalizing powered and product terms using residual centering, C. Sauer Stata tip 119: Expanding datasets for graphical ends, N. J. Cox