Forward Documentation¶

Forward is a bioinformatics utility to facilitate phenomic studies using genetic cohorts (i.e it was not designed for pheWAS studies based on electronic medical records). It was built with a strong emphasis on flexibility, performance and reproducibility. The documented interfaces make it easy for bioinformaticians to extend Forward‘s capabilities by writing their own implementations (e.g. to add support for a new file format or to optimize computation for their dataset) and the automatic reporting and archiving functionality greatly facilitate the dissemination and reproduction of results.

The different components of Forward. The customizable components have the tool icon.

Installation¶

To install Forward, simply run:

pip install forward


You can then run the tests using:

python -c 'import forward; forward.test()'

# Or if you want more verbosity:
python -c 'import forward; forward.test(2)'


Quick start¶

Configuration files¶

The easiest way to run a Forward experiment is to use the support for YAML configuration files. These files contain all the necessary information to define all the aspects of the experiment and serve as an archive to define what analysis was executed. They are automatically added to the interactive report.

An example can be found here:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Database: pyclass: ExcelPhenotypeDatabase missing_values: ["-9", "-99", "-88", "-77"] sample_column: "Sample" filename: /path/to/cohort_phenotypes.xlsx exclude_correlated: 0.8 Variables: - name: MyocInfarction type: discrete - name: BMI type: continuous transformation: log - name: BPSystolic type: continuous - name: PC1 type: continuous covariate: Yes - name: PC2 type: continuous covariate: Yes - name: GenderFemale type: discrete covariate: Yes Genotypes: pyclass: MemoryImpute2Geno filename: /path/to/data/impute2_extractor.impute2 samples: /path/to/data/forward_samples.txt filter_probability: 0.90 filter_completion: 0.95 filter_maf: 0.01 filter_name: /path/to/data/variants.good_sites exclude_samples: ["9210", ] Experiment: name: "ADCY9_forward" cpu: 4 build: "GRCh37" tasks: - pyclass: LogisticTest outcomes: all covariates: all - pyclass: LinearTest outcomes: all covariates: all 

The Database block (lines 1-6) represents the phenotype database that is used in the experiment. Multiple different pyclasses are availble to handle flat files or Excel files, but implementations of the phenotype database interface make it easy to extend this to other formats or databases. Some other options are also included in this example, such as the missing_values command that is passed to the underlying Python object to make sure that exclusions are properly represented. The sample_column directive which column contains sample IDs, filename is the path to the file containing phenotypes and exclude_correlated is used to mark the correlation threshold to exclude affected samples from the control groups of correlated outcomes. As an example of this last command, if angina and myocardial infarction are correlated at 0.8, individuals with angina will be excluded from the control group for myocardial infarction and the other way around.

The Variables block (lines 8-29) defines all the variables under study, including covariates. The name command should correspond to phenotype IDs from the Database section, the type command is used to identify continuous and discrete variables and the transformation command can be used to transform continuous traits (e.g. to achieve normality if the chosen statistical tests requires it).

The Genotypes block (lines 31-40) is used to represent genotypic data. For now, the included implementations of the genotype database interface are for micrarray data in plink binary format or for imputed IMPUTE2 files. Basic filtering of variants by imputation probability (filter_probability), by completion rate (percentage of non-missing genotypes for a given marker, filter_completion), by minor allele frequency (filter_maf) and by variant name (using a file with a single columns, filter_name) is also built-in for the MemoryImpute2Geno class. Note that the Memory in the class name is because everything will be loaded in memory, which means that it will be fast for small genetic datasets, but it won’t be suitable for larger datasets. Users are encouraged to either extract their region of interest or to implement a version that does supports indexing on the hard disk.

The Experiment block (lines 42-54) defines all the analyses that will be executed by Forward. The name is used as an identifier and the corresponding folder will be automatically created. If it already exists, Forward will refuse to run (because we don’t want to overwrite your data). The cpu instruction will be passed to the tasks and will determine how many parallel processes will be ran (if supported for the chosen tasks). The build (e.g. GRCh37) will be archived with other meta information to ensure reproductibility and could eventually be used in the interactive report. The tasks list is to tell Forward what statistical analyses are to be executed as part of this experiment. For now, only methods for common variant association testing are available (linear and logistic regression), but we are actively working on other statistical tests.

Running an experiment¶

To run the newly created configuration file, you can use the command line interface script (forward/scripts/forward-cli.py). Eventually, this will be automatically installed. The usage is simple, just pass the path to the configuration file.

forward-cli run my_configuration.yaml


A sample outpout will then look like:

INFO:forward.genotype:Loading samples from data/impute2/forward_samples.txt
INFO:forward.genotype:Setting the MAF threshold to 0.01
INFO:forward.genotype:Setting the completion threshold to 0.95
INFO:forward.genotype:Keeping only variants with IDs in file: 'data/impute2/chr16.imputed.good_sites'
INFO:forward.experiment:The build set for this experiment is GRCh37.
WARNING:forward.phenotype.db:Some samples were discarded when reordering phenotype information (1343 samples discarded). This could be because no genotype information is available for these samples.
INFO:forward.genotype:Built the variant database (17 entries).
INFO:root:Running a logistic regression analysis.
INFO:root:Running a linear regression analysis.

To view the interactive report, use the forward-cli script:

forward-cli report my_experiment


To view the generated interactive report, you can then follow the on-screen instructions:

forward-cli report my_experiment


and go to http://127.0.0.1:5000/forward with your favorite browser.

A sample report is available on StatGen’s website.

Also note that the interactive report is strictly optional and you can still browse the analyses results manually. See the next section for details.

Browsing results¶

After executing an experiment, the following directory structure will have been created:

└── My_Experiment
├── configuration.yaml
├── experiment_info.pkl
├── forward_database.db
├── phen_correlation_matrix.npy
├── phenotypes.hdf5


This contains all the results and information needed to describe the experiment. The only missing thing for perfect reproducibility is a copy of the input files. Eventually, an opt-out feature will allow users to have automatic archiving of the input files.

Here is a description of all of the results files.

• configuration.yaml The configuration file that was used to generate these results.

• experiment_info.pkl A Python pickle file contining experiment meta-data. See the following example for details

{'build': 'GRCh37',
'configuration': 'sample_experiment/experiment.yaml',
'outcomes': [u'Infarctus',
u'Valve',
u'Angine',
u'Diabete',
u'BMI',
u'BPSystolic',
u'BPDiastolic',
u'PC1',
u'PC2',
u'PC3',
u'GenderFemale',
u'Age'],
'phen_correlation': 'My_Experiment/phen_correlation_matrix.npy',
'phenotype_correlation_for_exclusion': 0.8,
'start_time': datetime.datetime(2015, 10, 4, 16, 15, 51, 680559),
'walltime': datetime.timedelta(0, 51, 395957)}

• forward_database.db A sqlite3 database containing all the results. Internally, Forward uses SQL Alchemy to create the database, this means that it will be easy to support more robust RDBMS without lots of changes to the codebase. A sample database will have the following tables:

• continuous_variables
• results
• discrete_variables
• variables
• linreg_results
• variants
• related_phenotypes_exclusions

See the documentation of forward.experiment.ExperimentResult for a full description of the schema for the results table.

• phen_correlation_matrix.npy A numpy binary file containing a correlation matrix for the outcomes. This is used to compute the exclusions based on related outcome correlation.

• phenotypes.hdf5 A HDF5 binary file containing all the data from the phenotype database. This is used by the report to create graphics on the fly, before and after transformations.

• tasks This is a subdirectory containing task metadata in the Pickle format.

This is the “low-level” alternative for browsing results from Forward experiments. Alternatively, if you dislike the web-based report but still want easy access to experiment results, you can use the forward.backend.Backend class directly from your own Python script.