Update README.md

README.md

@@ -19,13 +19,17 @@ The dataset has 3 columns:
 - `batch_idx`: the index of the batch to which a sequence belongs to (not present in the original dataset).
 - `token_ids`: the tokenised texts, each of length 2049 tokens.
 
-The dataset is split into 143 chunks (i.e., parquet files), each corresponding to the data seen by a checkpoint.
-The Pythia model checkpoints are available at initialisation (i.e., step 0) and each 1000 steps (i.e., steps 1000, 2000, etc) up to the last checkpoint (i.e., step 143000).
-We reflect this into the filenames: `train-001000.parquet`, `train-002000.parquet`, ..., `train-143000.parquet`.
+The dataset is split into 143 chunks (parquet files) and each chunk includes 1024000 sequences (rows) corresponding to 1000 batches, each formed by 1024 sequences.
+This means that each chunk corresponds to the data seen between between one checkpoint and the next.
+Specifically, the Pythia model checkpoints are available* at initialisation (step 0) and each 1000 steps (steps 1000, 2000, etc) up to the last checkpoint (step 143000).
+We reflect this structure into the filenames: `train-001000.parquet`, `train-002000.parquet`, ..., `train-143000.parquet`.
+Let's clarify the mapping between chunks and checkpoints with an example.
 
-**Example**: consider file `train-001000.parquet`. It contains sequences with `batch_idx` 0-999. These sequences where "seen" (i.e., the model took a gradient step on them) before taking checkpoint 1000.
+**Example**: Consider file `train-001000.parquet`. It contains sequences with `batch_idx` in [0, 999]. These sequences were "seen" by checkpoint 1000.
+Batches with `batch_idx` >= 1000 are only seen by later checkpoints.
 
-Note that additional log-spaced checkpoints are available for the initial part of training (i.e., steps 1, 2, 4, 8, ..., 512). I did not create a separate file for these, but you can easily subset the first chunk (i.e., `data/train-001000.parquet`).
+
+*NOTE: additional log-spaced checkpoints are available for the initial part of training (i.e., steps 1, 2, 4, 8, ..., 512). I did not create a separate file for these, but you can easily subset the first chunk (i.e., `data/train-001000.parquet`).
 
 
 ## License