Lasagna Plots Made in Different (Statistical) Ovens

To The Editor:

Interpreting repeated measures in longitudinal data analysis can be a challenge, especially in large studies and in studies with long follow-up periods. Our research team from the Breast cancer in Older Women study^1,2 struggled to figure out how to visually depict data on surveillance mammograms among a cohort of breast cancer survivors—with the overall goal of trying to determine whether surveillance mammography in older women is associated with decreased breast cancer mortality. We have long-term follow-up data on surveillance mammography among a cohort of 1859 breast cancer survivors. To make things more challenging, our outcome (breast cancer mortality) is dichotomous and follow-up is censored by other events (ie, breast cancer recurrence, mortality due to other cause, and disenrollment). With over 1800 women and 15 years of follow-up, spaghetti plots were incomprehensible.

It was with great interest that we came across the proposal by Swihart and colleagues³ in EPIDEMIOLOGY who proposed a new type of graph—the lasagna plot. The authors nicely explained the advantages of lasagna plots over spaghetti plots to display longitudinal data. Lasagna plots incorporate time (horizontal axis), values for repeated measures (colors or shadings), and space for displaying each subject (vertical axis) without overlapping data. Moreover, through various sorting and clustering strategies, lasagna plots make it easier to visually detect patterns of repeated measures in data.

In the article of Swihart et al,³ R code was provided to produce lasagna plots.⁴ However, our research team was not sufficiently versed in R to manipulate the code for our purposes. We contacted Dr. Swihart to find out if there was a code to create lasagna plots in other statistical packages. However, there was not, and so we set out to recreate these plots with SAS software (SAS Institute Inc., Cary, NC).⁵ We found that lasagna plot can be regarded as a nongeographic map using the GMAP procedure in SAS/GRAPH software. A grid is created as a “map”; values of repeated measures for each subject (represented by a specific color) are filled in for each corresponding unit on the grid; and labels and reference lines are added in the last step by annotations.

We duplicated the lasagna plots in Dr. Swihart’s commentary, and Dr. Swihart replicated the plots from their previously published data using our SAS code. The SAS code and the corresponding lasagna plots are available in an eAppendix (http://links.lww.com/EDE/A616). We are happy to provide this alternative baking option for lasagna plots for people like us with different kitchens.

Hongyuan Gao

Diana S. M. Buist

Group Health Research Institute

Seattle, WA

[email protected]

Timothy L. Lash

Department of Epidemiology & Prevention

Division of Public Health Sciences

Wake Forest School of Medicine

Department of Clinical Epidemiology

Aarhus University

Aarhus, Denmark

Jaclyn L. F. Bosco

Dana-Farber Cancer Institute

Boston Medical Center

Boston, MA

Bruce Swihart

Department of Biostatistics

Johns Hopkins Bloomberg School of

Public Health

Baltimore, MD