Originally designed application in the context of resource-limited
plant research and breeding programs, `waves`

provides an
open-source solution to spectral data processing and model development
by bringing useful packages together into a streamlined pipeline. This
package is wrapper for functions related to the analysis of point
visible and near-infrared reflectance measurements. It includes
visualization, filtering, aggregation, pretreatment, cross-validation
set formation, model training, and prediction functions to enable
open-source association of spectral and reference data.

Follow the installation instructions below, and then go wild! Use
`waves`

to analyze your own data. Please report any bugs or
feature requests by opening issues in the `waves`

repository.

Install the latest `waves`

release directly from CRAN:

Alternatively, install the development version to get the most up-to-date (but not necessarily thoroughly tested) version:

Match spectra with reference values so that you have a
`data.frame`

with unique identifiers, reference values, and
other metadata as columns to the left of spectral values. Spectral
column names should start with “X”. Remove rows with missing values.

```
ikeogu.2017[1:7, 1:7]
#> # A tibble: 7 × 7
#> study.name sample.id DMC.oven TCC X350 X351 X352
#> <chr> <chr> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 C16Mcal C16Mcal_1 39.6 1.00 0.488 0.495 0.506
#> 2 C16Mcal C16Mcal_2 35.5 17.0 0.573 0.568 0.599
#> 3 C16Mcal C16Mcal_3 42.0 21.6 0.599 0.627 0.624
#> 4 C16Mcal C16Mcal_4 39.0 2.43 0.517 0.516 0.514
#> 5 C16Mcal C16Mcal_5 33.4 24.0 0.519 0.548 0.554
#> 6 C16Mcal C16Mcal_6 32.1 19.0 0.576 0.566 0.589
#> 7 C16Mcal C16Mcal_7 35.8 6.61 0.530 0.536 0.525
ikeogu.2017.prepped <- ikeogu.2017 %>%
dplyr::rename(unique.id = sample.id,
reference = DMC.oven) %>%
dplyr::select(unique.id, dplyr::everything(), -TCC) %>%
na.omit()
ikeogu.2017.prepped[1:7, 1:7]
#> # A tibble: 7 × 7
#> unique.id study.name reference X350 X351 X352 X353
#> <chr> <chr> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 C16Mcal_1 C16Mcal 39.6 0.488 0.495 0.506 0.494
#> 2 C16Mcal_2 C16Mcal 35.5 0.573 0.568 0.599 0.593
#> 3 C16Mcal_3 C16Mcal 42.0 0.599 0.627 0.624 0.606
#> 4 C16Mcal_4 C16Mcal 39.0 0.517 0.516 0.514 0.536
#> 5 C16Mcal_5 C16Mcal 33.4 0.519 0.548 0.554 0.549
#> 6 C16Mcal_6 C16Mcal 32.1 0.576 0.566 0.589 0.591
#> 7 C16Mcal_7 C16Mcal 35.8 0.530 0.536 0.525 0.539
```

`plot_spectra()`

To display outliers in a different color, set
`detect.outliers`

to `TRUE`

.

`filter_spectra()`

.`waves`

uses Mahalanobis distance to identify outliers.
Mahalanobis distance is a common metric used to identify multivariate
outliers. The larger the value of Mahalanobis distance, the more unusual
the data point (i.e., the more likely it is to be a multivariate
outlier).

The distance tells us how far an observation is from the center of the cloud, taking into account the shape (covariance) of the cloud as well.

To detect outliers, the calculated Mahalanobis distance is compared against a \(\chi^2\) distribution with degrees of freedom equal to the number of spectral data columns and an alpha level of 0.05.

```
filtered.df <- ikeogu.2017.prepped %>%
filter_spectra(
df = .,
filter = TRUE,
return.distances = TRUE,
num.col.before.spectra = 5,
window.size = 15
)
#>
#> Removed 0 rows.
filtered.df[1:5, c(1:5, (ncol(filtered.df) - 3):ncol(filtered.df))]
#> unique.id study.name reference X350 X351 X2498 X2499 X2500
#> 1 C16Mcal_1 C16Mcal 39.62109 0.4881079 0.4951843 1.867465 1.870405 1.870702
#> 2 C16Mcal_2 C16Mcal 35.52017 0.5727389 0.5682541 1.901451 1.891114 1.888507
#> 3 C16Mcal_3 C16Mcal 42.04462 0.5989934 0.6266454 1.828793 1.826562 1.832022
#> 4 C16Mcal_4 C16Mcal 39.00999 0.5169374 0.5164186 1.836635 1.835856 1.834857
#> 5 C16Mcal_5 C16Mcal 33.44273 0.5189608 0.5477946 1.897076 1.899430 1.896130
#> h.distances
#> 1 139.8898
#> 2 149.5721
#> 3 140.5149
#> 4 148.3881
#> 5 152.2514
```

No outliers were identified in the example dataset. Note the if
`return.distances`

is set to `TRUE`

, the rightmost
column contains Mahalanobis distances (`h.distances`

).

If you have more than one scan per unique identifier, aggregate the
scans by mean or median with `aggregate_spectra()`

.

In this example, we will aggregate by `study.name`

.

```
aggregated.test <- ikeogu.2017.prepped %>%
aggregate_spectra(
grouping.colnames = c("study.name"),
reference.value.colname = "reference",
agg.function = "mean"
)
aggregated.test[, 1:5]
#> # A tibble: 2 × 5
#> study.name reference X350 X351 X352
#> <chr> <dbl> <dbl> <dbl> <dbl>
#> 1 C16Mcal 36.1 0.559 0.562 0.569
#> 2 C16Mval 36.4 0.545 0.549 0.552
```

`test_spectra()`

is a wrapper that performs spectral
pretreatment (5.1),
cross-validation set formation (5.2), and
model training functions over multiple iterations (5.3).

Note that the following subsections describe functions that are
called within `test_spectra()`

. They do not need to be used
separately for model pretreatment, cross-validation set formation, or
model training.

Some of the arguments for this function are detailed below. A
description of output is below under section 5.4. See `?test_spectra()`

for
more information on the arguments and output for this function.

```
results.list <- ikeogu.2017.prepped %>%
dplyr::select(unique.id, reference, dplyr::starts_with("X")) %>%
na.omit() %>%
test_spectra(
train.data = .,
tune.length = 3,
num.iterations = 3,
pretreatment = 1
)
#> Pretreatment initiated.
#> Training models...
#> Working on Raw_data
#> Loading required package: lattice
#> Warning: package 'pls' was built under R version 4.2.3
#>
#> Attaching package: 'pls'
#> The following object is masked from 'package:caret':
#>
#> R2
#> The following object is masked from 'package:stats':
#>
#> loadings
#> Returning model...
```

Specify which spectral pretreatments (1-13) to apply with the
parameter `pretreatment`

. `pretreat_spectra()`

can
also be used on its own to transform a data.frame using any/all of 12
available pretreatments:

- Raw data (no pretreatment is applied)
- Standard normal variate (SNV)
- SNV and first derivative
- SNV and second derivative
- First derivative
- Second derivative
- Savitzky–Golay filter (SG)
- SNV and SG
- Gap segment derivative (window size = 11)
- SG and first derivative (window size = 5)
- SG and first derivative (window size = 11)
- SG and second derivative (window size = 5)
- SG and second derivative (window size = 11)

```
ikeogu.2017.prepped[1:10, ] %>% # subset the first 10 scans for speed
pretreat_spectra(pretreatment = 2:13) %>% # exclude pretreatment 1 (raw data)
bind_rows(.id = "pretreatment") %>%
gather(key = "wl",
value = "s.value",
tidyselect::starts_with("X")) %>%
mutate(wl = as.numeric(readr::parse_number(.data$wl)),
pretreatment = as.factor(pretreatment)) %>%
drop_na(s.value) %>%
ggplot(data = ., aes(x = wl, y = s.value, group = unique.id)) +
geom_line(alpha = .5) +
theme(axis.text.x = element_text(angle = 45)) +
labs(title = "Pretreated spectra",
x = "Wavelength",
y = "Spectral Value") +
facet_wrap(~ pretreatment, scales = "free")
```

Note that the scales in this plot are “free”. Without free scales, anything derivative-based treatment (D1 or D2) looks like it’s a constant zero in comparison to those without derivative-based treatments (SNV, SG).

Choose from random, stratified random, or a plant breeding-specific
scheme from Jarquín et
*al.*, 2017. *The Plant Genome*. Options include:

`cv.scheme` |
Description |
---|---|

`NULL` |
Random or stratified random sampling (does not take genotype or environment into account) |

“CV1” | Untested lines in tested environments |

“CV2” | Tested lines in tested environments |

“CV0” | Tested lines in untested environments |

“CV00” | Untested lines in untested environments |

If `cv.scheme`

is set to `NULL`

, the argument
`stratified.sampling`

is used to determine whether stratified
random sampling should be performed. If `TRUE`

, the reference
values from the input `data.frame`

(`train.data`

)
will be used to create a balanced split of data between the training and
test sets in each training iteration.

When using one of the four specialized cross-validation schemes
(“CV1”, “CV2”, “CV0”, or “CV00”), additional arguments are required: -
`trial1`

contains the trial to be tested in subsequent model
training functions. The first column contains unique identifiers, second
contains genotypes, third contains reference values, followed by
spectral columns. Include no other columns to right of spectra! Column
names of spectra must start with “X”, reference column must be named
“reference”, and genotype column must be named “genotype”.
-`trial2`

contains a trial that has overlapping genotypes
with `trial1`

but that were grown in a different site/year
(different environment). Formatting must be consistent with
`trial1`

. - `trial3`

contains a trial that may or
may not contain genotypes that overlap with `trial1`

.
Formatting must be consistent with `trial1`

.

Cross-validation schemes can also be formatted outside of
`test_spectra()`

using the function
`format_cv()`

.

Many of the arguments for `test_spectra()`

are related to
model training: - `model.method`

is the algorithm type to use
for training. See the table below for more information -
`tune.length`

is the number of PLS components to test. This
argument is ignored if other algorithms are used -
`best.model.metric`

indicates the metric used to decide which
model is best (“RMSE” or “R-squared”) - `k-fold`

specifies
the number of folds used for cross-validation to tune model
hyperparameters within the training set - `num.iterations`

sets the number of training iterations - `proportion.train`

is the fraction of samples to be included in the training set (default
is 0.7)

Models can also be trained with the standalone function
`train_spectra()`

. Model training is implemented with `caret`

.

Algorithm | `model.method` |
R package source | Tuning parameters (hyperparameters) |
---|---|---|---|

Partial least squares (PLS) | “pls” | `pls` |
ncomp |

Random forest (RF) | “rf” | `randomForest` |
mtry |

Support vector machine (SVM) with linear kernel | “svmLinear” | `kernlab` |
C |

Support vector machine (SVM) with radial kernel | “svmRadial | `kernlab` |
sigma, C |

`test_spectra()`

outputs a list with four objects:

`model.list`

is a list of trained model objects, one for each pretreatment method specified by the`pretreatment`

argument. Each model is trained with all rows of the input`data.frame`

(`df`

)

```
summary(results.list$model)
#> Data: X dimension: 173 2151
#> Y dimension: 173 1
#> Fit method: kernelpls
#> Number of components considered: 3
#> TRAINING: % variance explained
#> 1 comps 2 comps 3 comps
#> X 62.65 68.38 91.17
#> reference 63.34 75.71 76.87
```

`summary.model.performance`

is a`data.frame`

containing summary statistics across all model training iterations and pretreatments. See below for a description of the summary statistics provided.

```
results.list$summary.model.performance
#> SummaryType ModelType RMSEp R2p RPD RPIQ CCC
#> 1 mean pls 2.0440540 0.77824381 2.085018 2.6038009 0.85155095
#> 2 sd pls 0.2661027 0.02093217 0.183391 0.3062501 0.04696515
#> 3 mode pls 1.8971232 0.78270780 2.139263 2.7700624 0.86628031
#> Bias SEP RMSEcv R2cv R2sp best.ncomp best.ntree
#> 1 -0.05516781 2.0652364 1.95463427 0.7755627 0.77670789 3 NA
#> 2 0.09468830 0.2688604 0.08412512 0.0157923 0.01211986 0 NA
#> 3 -0.13588174 1.9167831 1.96423744 0.7807426 0.76866372 3 NA
#> best.mtry
#> 1 NA
#> 2 NA
#> 3 NA
```

`model.performance`

is a`data.frame`

containing performance statistics for each iteration of model training separately (see below).

```
results.list$model.performance
#> Iteration ModelType RMSEp R2p RPD RPIQ CCC
#> 1 1 pls 1.897123 0.7827078 2.139263 2.770062 0.8662803
#> 2 2 pls 1.883812 0.7965839 2.235167 2.790961 0.8893859
#> 3 3 pls 2.351227 0.7554397 1.880623 2.250380 0.7989866
#> Bias SEP RMSEcv R2cv R2sp best.ncomp best.ntree
#> 1 -0.13588174 1.916783 1.964237 0.7807426 0.7686637 3 NA
#> 2 0.04906262 1.903334 2.033546 0.7578310 0.7708123 3 NA
#> 3 -0.07868431 2.375593 1.866120 0.7881145 0.7906476 3 NA
#> best.mtry
#> 1 NA
#> 2 NA
#> 3 NA
```

`predictions`

is a`data.frame`

containing both reference and predicted values for each test set entry in each iteration of model training.

```
head(results.list$predictions)
#> Iteration ModelType unique.id reference predicted
#> 1 1 pls C16Mcal_3 42.0446205 39.7341295535482
#> 2 1 pls C16Mcal_11 35.2299995 37.0584291524637
#> 3 1 pls C16Mcal_14 42.2379684 41.2378709425078
#> 4 1 pls C16Mcal_16 36.3796349 38.4719648775467
#> 5 1 pls C16Mcal_17 36.6281929 38.3893405413672
#> 6 1 pls C16Mcal_21 37.6122742 37.5876338455539
```

`importance`

is a`data.frame`

containing variable importance results for each wavelength at each iteration of model training. If`model.method`

is not “pls” or “rf”, this list item is`NULL`

.

```
results.list$importance[, 1:7]
#> # A tibble: 3 × 7
#> Iteration ModelType X350 X351 X352 X353 X354
#> <int> <chr> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 1 pls 0.0258 0.0245 0.0278 0.0268 0.0265
#> 2 2 pls 0.0351 0.0344 0.0329 0.0315 0.0328
#> 3 3 pls 0.0241 0.0251 0.0256 0.0258 0.0253
```

Statistic* | Description |
---|---|

RMSE_{p} |
Root mean squared error of prediction |

R^{2}_{p} |
Squared Pearson’s correlation between predicted and observed test set values |

RPD | Ratio of standard deviation of observed test set values to
RMSE_{p} |

RPIQ | Ratio of performance to interquartile difference |

CCC | Concordance correlation coefficient |

Bias | Average difference between the predicted and observed values |

SEP | Standard error of prediction |

RMSE_{cv} |
Root mean squared error of cross-validation |

R^{2}_{cv} |
Coefficient of multiple determination of cross-validation for PLS models |

R^{2}_{sp} |
Squared Spearman’s rank correlation between predicted and observed test set values |

best.ncomp | Best number of components in a PLS model |

best.ntree | Best number of trees in an RF model |

best.mtry | Best number of variables to include at every decision point in an RF model |

*Many of the spectral model performance statistics are calculated
using the function `postResampleSpectro()`

from the
`spectacles`

package.

`save_model()`

- Intended for a production environment
- Can evaluate spectral pretreatment methods using the input dataset
- Selects best model using the metric provided with
`best.model.metric`

(“RMSE” or “Rsquared”) - Returns trained model with option to save as .Rds object
- The
`$model`

output from`test_spectra()`

can also be saved and used for prediction, but`save_model()`

will take the extra step of saving an .Rds file for you if`write.model`

is set to`TRUE`

.

In the example below, we’ll use one subset of the example dataset (“C16Mcal”) to create the model and then we’ll predict the other subset (“C16Mval”) in section 7.

```
model.to.save <- ikeogu.2017.prepped %>%
dplyr::filter(study.name == "C16Mcal") %>%
dplyr::select(unique.id, reference, dplyr::starts_with("X")) %>%
na.omit() %>%
save_model(
df = .,
write.model = FALSE,
pretreatment = 1:13,
tune.length = 5,
num.iterations = 3,
verbose = FALSE
)
```

Now let’s take a look at our trained model:

```
summary(model.to.save$best.model)
#> Data: X dimension: 120 2141
#> Y dimension: 120 1
#> Fit method: kernelpls
#> Number of components considered: 5
#> TRAINING: % variance explained
#> 1 comps 2 comps 3 comps 4 comps 5 comps
#> X 64.48 87.94 91.43 95.16 97.75
#> reference 33.93 64.97 87.18 89.26 89.54
```

```
model.to.save$best.model.stats %>%
gather(key = "statistic", value = "value", RMSEp_mean:best.mtry_mode) %>%
separate(statistic, into = c("statistic", "summary_type"), sep = "_") %>%
pivot_wider(id_cols = c(Pretreatment, summary_type),
names_from = statistic, values_from = value)
#> # A tibble: 3 × 15
#> Pretreatment summary_type RMSEp R2p RPD RPIQ CCC Bias SEP
#> <chr> <chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 SNVSG mean 1.49 0.886 2.82 3.66 0.930 0.0622 1.52
#> 2 SNVSG sd 0.0499 0.00219 0.125 0.208 0.00587 0.605 0.0506
#> 3 SNVSG mode 1.55 0.885 2.74 3.49 0.926 -0.624 1.57
#> # ℹ 6 more variables: RMSEcv <dbl>, R2cv <dbl>, R2sp <dbl>, best.ncomp <dbl>,
#> # best.ntree <dbl>, best.mtry <dbl>
```

If generating predictions from a saved model file in .Rds format, use
`predict_spectra()`

. If the model object is already in your R
environment, the function `stats::predict()`

can be used to
generate predictions. `predict_spectra()`

pulls the best
model hyperparameters from your saved model object, but if using
`stats::predict()`

, these must be supplied separately.

Using the model we trained in section 6, we can predict cassava root dry matter content for our held out validation set:

First, determine which pretreatment generated the best model. In this case, it’s “SNVSG”, which is pretreatment #8. Pretreat the new spectral dataset with these spectra.

```
pretreated.val <- ikeogu.2017.prepped %>%
filter(study.name == "C16Mval") %>%
pretreat_spectra(pretreatment = 8)
pretreated.val.mx <- pretreated.val %>%
dplyr::select(starts_with("X")) %>%
as.matrix()
best.ncomp <- model.to.save$best.model.stats$best.ncomp_mode
```

```
overall.range <- c(min(c(pretreated.val$reference, predicted.values)),
max(c(pretreated.val$reference, predicted.values)))
cbind(unique.id = pretreated.val$unique.id,
observed = pretreated.val$reference,
predicted = predicted.values) %>%
as_tibble() %>%
mutate(observed = as.numeric(observed),
predicted = as.numeric(predicted)) %>%
ggplot(aes(x = observed, y = predicted)) +
geom_abline(intercept = 0,
slope = 1,
color = "gray80") +
geom_point() +
coord_fixed(xlim = overall.range,
ylim = overall.range) +
labs(title = "Example dry matter content predictions",
x = "Observed",
y = "Predicted") +
theme_bw()
```