Identify nomenclature that unambiguously describes the components of matrix-api objects

[ambrosejcarr]

This issue identifies confusion with the sc-group and sc-dataset nomenclature and contains a discussion about potential replacement names and how those names should be propagated to the broader feature-object-matrix standardization efforts.

[ivirshup]

I find the current nomenclature (sc-group :: AnnData, sc-dataset :: MuData) a little confusing.

I'd thought the names were switched, since sc-group sounds more plural than sc-dataset to me. There's also some ambiguity when multiple studies are collected together, since dataset could refer to either. One suggestion would be sc-collection for the plural.

[falexwolf]

I also stumbled across the naming when first reading it in the Google Doc proposal and made a comment. Months later, I personally got used to it and can't think of a better alternative.

I do think it'd be helpful to mention the naming convention translation (sc_group~AnnData~assay~..., sc_dataset~MuData~...) in the docs for those readers familiar with this convention, and introduce these as the most high-level terms first in the specification of the schema.

Here comes a suggestion along these lines: #17

[ivirshup]

I still feel like using group for singular and dataset for plural can be confusing.

Especially when HDF5 and zarr, which one might refer to in the same breath, define Groups as a collection of Datasets. I feel like potential confusion here could be avoided by just picking a different name. (sc-group :: AnnData, super-group :: MuData) would also avoid this. Though there is still the issue of "wait, which kind of 'group' do you mean?".

[falexwolf]

I'd be very much open for a different naming choice, but I also think that would require a more in-depth decision doc.

For instance, I'd feel "super-group" bears more potential for confusion in the future.

[ambrosejcarr]

@ivirshup do you think this would be mitigated in practice when schema specifications + group naming is applied in implementations of the schema? I feel they'll make things more concrete.

I'm open to changing the name, but I think Alex's changes in #17 mitigate most of the confusion and wonder if we're in an area of diminishing returns.

As an example, the TileDB implementation we're working on has the following structure. This example models a 10x multiome.

pbmc_small  # sc_dataset
├── __tiledb_group.tdb
└── RNA   # sc_group
   ├──── __tiledb_group.tdb
   ├──── var
   ├──── obs
   ├──── obsm
   ├──── varm
   └──── X
└── ATAC  # sc_group
   ├──── __tiledb_group.tdb
   ├──── var
   ├──── obs
   ├──── obsm
   ├──── varm
   └──── X

[ivirshup]

I think part of the issue is me working with very concrete examples (hdf5 and zarr APIs) where groups contain datasets. The context switch here keeps tripping me up. I'm not sure I've had a day where I've thought about this and not mixed them up at least once.

I don't think this is a huge sticking point for me, but I think we're going to have some confused conversations if the current sc-dataset, sc-group convention stays. If one of these names changes, I think we can avoid that.

[falexwolf]

I understand Isaac's discomfort with the naming choice and have given it more thought to come up with a potentially better choice. To that end, I'll try to offer an additional perspective on the current specification.

What we are discussing here is the naming choice of two "data structures" giving rise to two "data container objects" and their serialized counterparts. Here comes a table with a few naming suggestions. Please note that the new random abbreviation SLAM is just meant to ditch all connotations with existing terms and could be any acronym for that sake.

Data structure	Possible names
A simple layered annotated matrix (SLAM) storing measurements of variables.	sc_group, sc_matrix, slam, sla_matrix (AnnData)
The overall container structure, a collection of related SLAMs.	sc_dataset, sc_collection, sc_data, omics_data, slam_collection, slam_data, sla_collection (MuData)

To jump to my conclusion for selecting names: My strongest inclination is that dataset is indeed too confusing of a term as it has several opposing specific meanings in other instances, like the ones Isaac mentioned.

I'd suggest replacing it with another term. My favorite would be a new term like sla_collection or similar for these reasons:

• "Collection" is the name of the main distribution unit for the cellxgene data portal. Other single-cell portals should feel comfortable with the name, too.
• "Collection" is the name of the most low-level container library in python dealing with unstructured (flattened) collections of items.
• "Collection" alone is too generic of a term, hence there needs to be a prefix.
• The prefix could be sc due to wide adoption and use of that prefix. However, measurement technologies that characterize single nuclei or spatial barcoded spots or pixels do not characterize single cells. There might be more such technologies in the future, and hence I'd favor a prefix that merely aims at describing the data structure. Hence, for instance, sla_collection.

I'd also suggest replacing sc_group as group has strong opposing connotations both to biologists ("group of cells") and in software (zarr, tiledb, hdf5), where it typically means "collection/folder of arrays=datasets". I'd suggest sla_matrix for similar reasons as above. Of course, the precise name doesn't matter so much, it could be a different acronym. But I think it'd be nice if the name encoded the meaning of the data structure.

I'll leave it with this first take on it.

I'd love to hear more in-depth perspectives on naming rationales for the two data structures! And I'd encourage us to still consider the current names sc_group, sc_dataset placeholders until a few more people have commented. It's a very fundamental decision!

---

For more background, for anyone who still wants to read on: Here comes an effort to define a SLAM¹:

• The first array index (row index) represents observations. An observation is meant to characterize a single cell, a nucleus, a barcoded spot, a pixel, or a similar observational unit. The actual measurement might not always represent the desired system due to measurement noise. ²
• The second array indices variables that are jointly measured. A joint measurement means: measured in the same system and measured at the same time. In case there is ambiguity of what a system is, assume the smallest observational units (a cell, a nucleus, a barcoded spot, pixel as opposed to a biological sample, tissue, etc.).
• "Simple matrix" means no additional structure neither on observations nor variables, i.e., observations & variables have the same type & unit.
• "Layered matrix" means there may be multiple measurements with the exact same shape associated with different states of variables of the same type & unit: for instance, spliced vs. un-spliced RNA.³
• "Annotated matrix" means comprehensive annotations exploiting conserved dimensions following a defined naming schema such as the one in AnnData.

And here comes an effort to define "A collection of related SLAMs."⁴:

• "Related" means that SLAMs share much or all of the (system, experiment) metadata coming with a batch of measured data. Thus, a collection bundles all information on a given biological system under given conditions, which is the primary concern of downstream users that try to extract value from the data.
• "Related" also means that metadata differences across SLAMs in a collection are merely due to differences in (readout, technology, comp-workflow).
• As presently defined, all SLAMs within a collection have the same observational unit. Hence, we do not mix, for instance, single-cell and bulk data.
• A collection can map a significant portion of the data distributed through academic studies & joint-multi-modal assays (e.g., 10x Multinome). In the present form, it fails to capture orthogonal data indexed by different observational units (bulk, imaging, flow, etc.). Nonetheless, a collection can evolve to the natural distribution format by data portals such as the cellxgene data portal.

Footnotes

1. A SLAM is a meaningful way of structuring & storing data for contingency reasons and typical downstream use cases (queries & learning algorithms): https://github.com/single-cell-data/matrix-api/issues/3#issuecomment-1061719252 ↩

2. The data structure is agnostic to that and would represent such information ("empty cells", "doublets", "dead cells", ...) through annotation. ↩

3. Operations on layers of a matrix are particularly simple as dimensions are conserved. Simple aggregations like summing up different layers typically produce meaningful summary statistics (for instance, total counts across spliced and unspliced). ↩

4. Modern biology characterizes different types of systems across different types of readout variables ("dimensions of biology", "features of biological systems"). Systems are sourced & treated with different experimental protocols, and variables are measured with different types of readout technologies. Readout data from the measurement device is then processed across different computational workflows to bring them into matrix format. The detailed semantics of the five-tuple ("system, experiment, readout, technology, comp-workflow") is beyond the scope of the present data structures. However, it is clear that it is not matrix-like. The simplest structure that could map arbitrary complexity within the four-tuple is a flat list, where each item in the list represents an observation of the metadata in the four-tuple. In this case, often, the observational unit is a sample or a batch of homogeneously processed samples. Multi-modal measurement technologies allow sharing much or all of the metadata in ("system, experiment") across SLAMs which makes it attractive to store a collection of SLAMs together. ↩

[joshua-d-campbell]

This is something I struggled with in the FOM schema as well. Maybe the term "matrix_group" could refer to a set of matrices that are related and "dataset" could be a collection of matrix groups. If the matrices within a sc_group can have different numbers of features, then it might be better to call it an "obs_group". The only other generic possibility that I can think of is "subset" or "modaility_subset" since each sc_group is a unique combination of modalities, observations, and features.

[ambrosejcarr]

Thanks very much for the clarity about the challenge @ivirshup in-depth decomposition of this question @falexwolf

We've discussed and favor the replacement of _dataset with _collection for the reasons you outline, and agree that it would reduce confusion. We also agree that any of these concepts are too generic by themselves, and require a suffix.

Our discussions lead us to propose two additional requirements on the suffix, since we're going to say it a million times: It should be pronounceable, and spellable. slam_ would meet these requirements, sla_ would not.

Our concern with slam is whether "matrix" is unnecessarily limiting. I've thought about it and don't see any rationale for > 2 dimensions (observations, features). We could have made the decision to create a tensor instead of the sc_dataset / sc_group division but because each group is separately processed and filtered, we felt that would introduce strange dependent sub-layers into the object, so did not use that approach. It would also have been harder to transform into toolchain models.

I'm interested to know what level of confidence @falexwolf and @ivirshup have that "matrix" is not overly limiting. If it's not, then slam and slam_collection are reasonable.

We're also interested in ideas for alternative acronyms, and agree we should let this issue germinate a bit before making the change.

[joshua-d-campbell]

Sorry, I didn't refresh to see @falexwolf's last post before adding my own. I also like slam as a potential name. The "simple" does not seem overtly necessary to me (I'm not sure what a complex lam would be at this stage), so it could be shortened to lam to get it to 3 letters like many file formats. It could also be reversed to mal (Matrix with Annotations and Layers).

I don't really have a strong opinion, but I do wonder whether it is worth aligning the name with FOM working group. The FOM (feature observation matrix) acronym was thought up quickly for a grant last summer to get the working group going. I asked during our first call if anyone had other thoughts but didn't get much of a response. slam, fom, and matrix-api may all be different things and should have different names, but I get worried that groups we are trying to work with will get confused if it all sounds too similar, especially if the differences between all of them are subtle.

[falexwolf]

I strongly agree with @joshua-d-campbell's point on aligning FOM vs. matrix-api vs. the data structure name.

To @joshua-d-campbell's point about "simple": I think people might (and I know did) construct matrices by joining, for instance, RNA & ATAC measurements into one contingent table. To me that's then no longer a simple matrix in this sense:

"Simple matrix" means no additional structure neither on observations nor variables, i.e., observations & variables have the same type & unit.

I'd discourage storing "such a complicated object" over a "simple one" as users would have a hard time understanding it.

I agree with @joshua-d-campbell that we don't need to have "simple" in the name. A specification in the docs that discourages storage of intermediate-stage-processed RNA & ATAC could address the above concern!¹ I think that having or having not "simple" in the name is more an aesthetic consideration.

To @ambrosejcarr's point of whether the term "matrix" is limiting.

My thinking was as follows: Yes, a "data matrix" is an array of dimension 2 ("tensor of degree 2"), with index-identifications observations ⨉ features.

There may be measurements that generate data in meaningful higher-dimensional structures that could be worth absorbing in a higher-dimensional array.

The only candidate I can presently think of is spatial data. But to take away the conclusion: I think also spatial data should be stored in annotated-matrix form within the scope of the present data structure specification. Below comes why I think so.

I'm not an expert, but as far as I know, there are two common ways of representing spatial information:

1. Storing it annotated-matrix-like: each observation is annotated with a 2 or 3-dimensional spatial coordinate. There is an arbitrarily high number of "primary/molecular measurement dimensions", e.g., genes from the whole transcriptome, a few selected genes/stains/probes, an arbitrary number of colors, etc. These measurement dimensions are often called channels.
2. Storing it as a collection of images: many devices - including cameras, medical imaging, high-throughput microscopes, etc. - produce data that is stored as a folder of image files. In regular image files, every pixel comes with measurements in four channels (RGBA). In assays like cell painting and related it may be many more channels, where each channel has a biological meaning according to the stain/probe. After processing a folder of image files, for regular images, to my knowledge, it's common practice to store them as 4- or 5-dimensional arrays/tensors with this layout (observation, x_index, y_index, z_index, channel_index). To arrive there, at least some of resolution adjustment, interpolation, reshaping, and index definition was applied to define the data on regular 2d or 3d grids. Hence, likely, this tensor doesn't contain raw data anymore but (coarse-grained) interpolated values. The big advantage is that the data is now ready for 2d- and 3d convolution. In a flattened layout where (x_index, y_index, z_index) are all adjacent in some form, this wouldn't be the case.

We see things got a little complicated in 2!

I generally wonder whether one shouldn't favor 1 as a first "go-to" stored representation, in particular as this also seems what OME now considers (ome/ngff#64). There are many ways of putting measurement data on a grid through reshaping, interpolating, adjusting resolutions, etc. These are highly non-trivial ML-related comp-workflows. Most of them won't fit into the metadata model that we're discussing here.

If people do use imaging data, I'd encourage them to store point measurements that are annotated with spatial information as an intermediate representation after a "folder of files". This would fit the present scope of the present repository and be compatible with the "annotated matrix" layout. Reshaped and further-processed tensor/array like representations of the same data could then be dealt with ML data infrastructure. I'm pretty sure there should be solutions for this but would have to investigate.

This whole discussion is related to how much the present format should be seen as a "canonical intermediate format" for omics data as opposed to a format that can also absorb all potential downstream representations. Given the reasons above (with really the dominant reason being the ability to build powerful metadata-schema specification around a matrix), I think it should be considered the former: Upstream (fastqs, folders of images, etc.) and downstream (highly-processed tensor-like data) formats that are non-annotated-matrix like should be handled elsewhere.

One should maybe stress this: Restricting ourselves to "annotated matrices" does not mean any information loss. It means that in some cases some nd-array structure that could be absorbed within the substructure of a flat "primary molecular/measurement dimension" would have to be flattened. But in my mind,

1. such substructure will only occur downstream
2. such substructure would complicate defining a broader metadata schema specification and hence broaden the scope of the present data structure too much

Sorry, I hope this didn't end up being too convoluted. Please let me know if it is and I'll try to streamline the writeup. 😅

Footnotes

1. I'd consider a joint embedding learned across RNA & ATAC again "simple" as its features would correspond to the homogeneous, unstructured output dimensions of a computational model. The computational model absorbed the complexity of dealing with the structure RNA vs. ATAC in the input and subsequent model architecture. ↩

[ivirshup]

I've thought about it and don't see any rationale for > 2 dimensions (observations, features).

One case here is genomics data. 3 dimensional arrays used by sgkit (observation, feature, alleles). My understanding here is that it's important for the central data representation to have ploidy information, but that dimension is dropped for most associated arrays.

For spatial data having pixel and point level representations, I agree these have fairly different use cases. If these representations are in scope, a BED file like representation for ATAC data would be in scope as well.

I do think it's important that data where point and read level information is associated with an annotated matrix is compatible here. But my current thinking is that the solution there is essentially to keep their obs x var annotated matrices compatible with matrix-api sc_groups and "symlink them" into sc_datasets (or whatever the terms will be)